There are some great resources out there on brahmavihara meditation. One is Sharon Salzberg's book Lovingkindness, The Revolutionary Art of Happiness. Each chapter contains explanations and a few anecdotes illustrating the subject of that chapter (for example, how to deal with anger and resentment towards people by using metta practice). The end of each chapter contains the actual instructions for how to do the meditation.
Some of the resources are on the internet.
A somewhat technical article, describing an intense version of the brahmavihara practice.
http://www.buddhanet.net/mettab5.htm A document that describes the brahmaviharas in rich detail.
http://www.vipassana.com/meditation/four_sublime_states.php Basically, brahmavihara practice is pretty simple. You just cultivate the specific feeling/attitude you're working on, and then you focus on it and try to grow it until it becomes more and more expansive. First your heart overflows with it, then your entire body, then the room you're in, then the entire area near you, then the earth, and then you fill the entire universe with it.
It usually starts by reciting a phrase such as "may all beings be happy and become awakened" as genuinely as you can. Of course, if you could feel the sentiment strongly enough to fill the entire universe with love the first time you said it, it wouldn't be a practice. So expect it to feel somewhat awkward at first, and trust that the sentiment develops and grows with practice. That's how it goes. The results of pregaming for psychedelics with brahmavihara practice are immense.
Yes, it takes time to become an anagami. There are plenty of cases of people in the hardcore dharma scene doing the first two paths in a year's time, especially with the aid of retreats, but I haven't personally heard a case of someone doing the first 3 paths in a year.
At the risk of making this post overly long and filled with imperfect advice, I'll give my recommendation with regards to the insight path and psychedelics, because I just don't see this information anywhere else. Before I give the recommendations, let me confirm your understanding of the general advice concerning psychedelics + insight meditation: you're exactly right that it moves you into some of the territory, but doesn't necessarily give you any of the skills to handle it. Then again, it can't always be said that vipassana itself gives the skills to handle the territory optimally either; it just moves you through to the other side of it as long as you keep using the technique correctly.
Now, if you're actively dark nighting at the time of an ayahuasca ceremony, I recommend against it. Not out of a fear of you seriously damaging yourself; it's just really unpleasant and it's hard to learn anything from what appears to be meaningless extreme pain. But if it's your only opportunity to share such an important experience with these people who mean so much to you, then fine. Every cloud has a silver lining, and you'd be expected to bond a LOT with whoever you do it with.
If you've just finished a path and you're still in the early review of that path, I think that would be the best time -- but who can plan that? "Okay, the ceremony is on the 20th. I guess I'll need to schedule my 2nd path moment for the 15th, around noon..." Still, if it works out like that, it would be awesome!
If you're in the early stages of a path, like the Mind and Body through Three Characteristics zone, it's reasonable to expect the A&P to get triggered by the ceremony, though it isn't
at all certain. Still, if it happens, try and solidify the joy of the A&P and/or the cool, peaceful bliss of Dissolution into the corresponding second and third shamatha jhanas, respectively. The reason for that is to avoid getting into the dark night territory during the ceremony. It's just not skillful to let yourself get into a new Fear, Misery, or Disgust stage under the influence of long-lasting DMT.
If you're in Equanimity, then the decision is up to you. If you're really solidly into Equanimity, you'll probably be fine. If you're just barely in Equanimity, then expect to get yanked back down into the later dark night stages.
Please let me know if any of the above is unclear or unhelpful. I'm a bit sleep deprived and I don't know if I'm talking any sense today.