There are only a few suttas where the Buddha directly discusses technique, the Saṅkhitta Dhamma Sutta is one of them

Daniel Ingram cites MN 111 as his favorite sutta and it's pretty easy to see why this may be. It gives a clear and concise account of the techniques someone follows to take them all the way to arahant, namely emerging from each of the jhanas in turn and investigating their causes and conditions. The Sankhitta sutta is similar, but ties together jhana, brahamaviharas, the foundations of mindfulness, and dhamma factors (eg the 7 factors of enlightenment) explicitly. I think the contents of this less than 10 paragraph sutta are invaluable, but there's a problem. These suttas are great if you already know what they're talking about, but they're so laconic that they're often inscrutable.

A debt is owed to translator Piya Tan and the translation I am working from can be found here. Also note that I'm sure my explanation would be considered idiosyncratic by many standards, please take with the standard grain of salt and as fuel for your own curiosity. 

First, briefly, an outline/map of the proposed practice:

Someone comes to the Buddha and asks for a simple description of his system. The Buddha says okay, but to please take seriously what he's about to say.

The meditator is instructed to cultivate the jhana factors and set aside the hindrances to jhana.

Taking these qualities as the basis, the meditator then applies them to the brahmaviharas, loving-kindness, compassion, appreciative-joy, and equanimity.

Taking equanimity as the basis, the meditator applies this clear state of seeing to the four foundations of mindfulness in turn: 

The Body

Feeling Tones

Mental Formations

Dhamma Factors

The last one might sound a bit more ambiguous, since it potentially gives us a vast array of mental phenomena to investigate, but we have help from another sutta[1] where Brahmanviharas as a practice are discussed. In that sutta, petitioners come to the Buddha Sariputta and ask him about his spiritual system. They say other teachers teach the Brahmaviharas and ask the Buddha Sariputta what his opinion is on these as a practice, and if such a practice will take one 'all the way.' The Buddha Sariputta says Brahmaviharas are great, but won't take you all the way. But if you cultivate the Brahmaviharas and then cultivate seeing the three marks in the five skandhas (dhamma factors) this will take you all the way.

I very much appreciate us having direct sutta evidence for practice outcomes as the concatenation of skills that can be developed in our formal practice. It is also nice to see direct instructions to take Brahmaviharas as concentration objects, since elsewhere they are not discussed as formal practice directions.

What follows is some thoughts on each of the outlined steps.

Five hindrances to jhana:

Hindrances (desire, aversion, ill-will, torpor, doubt) are usually discussed in the context of suppression. This suppression is temporary, but people still have a lot of difficulty with it. I find it helpful to investigate the hindrances as the side effects of parts not really having any confidence in meditation practice, or alternative priorities. They are trying to get you to pay attention to other good things. So using Core Transformation (book link) to spend some quality time with hindering thought patterns is a more long term solution IME, for dealing with the hindrance acutely in the moment, for building up internal trust, and for learning moves that wind up being helpful elsewhere in practice as well. This also helps when you deploy the various antidotes to hindrances that other practices recommend, as this can then be done more gently, with an eye towards solving the underlying problem rather than more internal violence. This generally leads to a substantial amount of insight.

Jhanic factors:

The jhanic factors are directing attention, sustaining attention, physical and emotional tranquility/contentment, physical sensations (generally pleasant though not uniformly so), and one-pointedness of mind. I'm a big fan of deliberate practice via getting as specific as possible with the mental moves that cultivate each factor. So for instance rather than waiting to forget about initial directing of attention, you can just intentionally direct your attention over and over again to the meditation object, followed by intentionally letting your attention relax to other objects. A practice given to me was to set a timer to ding every thirty seconds so you can get 20 reps in in a 10 minute practice. Similarly with sustained attention, the practice given was to actively think about the meditation object (including internal verbal reasoning about it, often derided or taken as evidence of doing the practice wrong) for about a minute at a time before relaxing for a bit then doing it again. Trying this sort of exercise, you may start to notice that many practice regimes are trying to start out well beyond your present abilities. Tranquilizing the bodily sensations and cultivating positive emotions are likewise something that can be started as a micro-practice of up to a minute at a time for a few rounds before relaxing and only building on this as skill is picked up and momentum develops on its own. One-pointedness of mind naturally begins arising on its own as the other four factors are developed.

For both the hindrances and factors it is considered useful to review at the end of your sit your guesses as to whether each of them was present. The point of this is not to be 100% accurate but to develop a habit that spins up a little sub process that pays just a bit more attention on the margin and will passively upgrade some related mindfulness skills as a bonus.

It might be more obvious, given this, why a simpler meditation object is often helpful. A candle flame, sensations of the breath, or even a colored disk, reduces the number of things you're attending to in your practice while trying to track all these factors and hindrances.

How and why to take the brahmaviharas as meditation object:

Doing this the way it is instructed with mantras or calling up the physical sensation raw never worked for me. What did work was doing Core Transformation until a core state was reached, and then calling in jhanic factor skills that had been previously cultivated to use that core state as the meditation object. A bonus is that since I had literally just done the work to address any objections or hindrances to feeling that core state in the CT session, it was drastically easier. As for why the brahmaviharas/core states make great meditation objects, the choice of object seems related to how fundamental the related physiological building blocks of the object are. There are an uncountable number of objects you could choose to experiment with, but most of them are compound mental objects, and will fall apart quickly as you traverse layers of the perceptual stack. Even the breath disappears at a certain point in altered states (which often causes people to break out of the state until they get used to it). We're taking it on the word of people who have been farther than us that the brahmaviharas don't break down. In fact, they are named as the qualities that remain even for a fully awakened Buddha. So that's pretty nice. In the same way that the jhanas when cultivated tend naturally towards the fourth jhana, Equanimity, so too the brahamaviharas naturally tend to evolve towards Equanimity, which can then be used for the next step.

Emerging from equanimous states and turning attention to the foundations of mindfulness:

 

What is it about equanimity that is so helpful for investigation? One way to think about it is that being non-judgemental about what you're seeing is exactly the attitude you'd want from an investigator. An investigator who was already sure about what he was seeing and what it meant would be an inquisitor. Secondly, when equanimous, mental events happen, and then, importantly, stop. They don't keep daisy chaining to the next thought with your knee jerk reaction to the previous one. This is helpful when trying to figure out what's happening moment to moment. 

 

Investigating the body is pretty straightforward, most people have heard of a body scan, and you can look up guided body scans. Investigating feeling tones is investigating the way things show up in consciousness as positive, negative, or neutral, and our involuntary reactions to those. Investigating the mental formations is a bit trickier, so it's great if we've built up some skill by this time. Mental formations tend to chain into one another very easily, and we want some clarity around exactly how that cause and effect goes. What are the textures of the mental images and mental talk? Observing how they are associative in nature and how the body reacts to them (which then is fuel for new mental formations etc).

Often, the practices leading up to this point will already be causing lots of insights and shifts, which is great, and now the practice can turn towards what Rob Burbea would term 'insight ways of looking' directly, with dhamma factors. This is much easier now that the sub skills have been built up. Many books have been written on this topic but an easy trailhead is anicca, or impermanence. With clarity it is now easy to see that there's a weird tension built in to attention. Attention wants to attend to stable qualities yet attention's very nature is this fast flickering thing that can't really stabilize except through great effort, and even then very temporarily. When you do manage to stabilize it on something, that something tends to disappear or otherwise deconstruct. What's going on? 😉

From here we're back to the regular insight meditation story, but with enough resolution to really get somewhere in a reasonable amount of time. Cultivate dhamma factor ways of looking, then turn it back on all the structures we've just learned to cultivate attention for, mental formations, feeling tones, body, jhanic factors, hindrances, attention itself. At some point interesting things start occurring. If these interesting things are emotionally disruptive, you have some facility with an integration practice like Core Transformation that should help out quite a bit, and any access you have to pleasant jhana states will also ameliorate stress/nervous system disruption. If/when unpleasant states arise, the strategy is the same, observing them with equanimity and any dhamma factors one has access to.

 

Conclusion:

 

This turned into a bit of a beginner orientation post in some ways. But my main motivation is that I think it's a really big deal that we have a sutta tying together brahmaviharas (often interpreted as Sila, moral training), samadhi (concentration), and insight/mindfulness practices (investigating dhamma factors leading to advancement on the path model) in a mechanistic/technique focused way. Many people have independently reinvented some or all of this style of approach in relatively modern times (since late 19th century) but I have never seen the Sankhitta sutta brought up as supporting evidence. I hope that by alerting people to this, AN 8.63, some people will find benefit.

May you realize freedom.

[1]: noble friend Andrey pointed out that this is a commentarial conflation of the Metta Sutta and the Silvant Sutta. In Buddhadharma, Bhikkhu P.A. Payutto points out that the three marks often co-occur when the five aggregates are mentioned, making this a reasonable practice inference.