Although I have dronk ayahuasca for over 4 years, and a period pretty frequent (about 1x or 2x a week) I have never brewed it myself, because up to now I've been drinking my aya in the Santo Daime context.
My experience with the mix they prepare is that I just dont have that much purging on a physical level as some have, but maybe i have a strategy that works good....
Next to that, the precise mix of plants seems to have a lot of impact on the purging aspect. So maybe I have been just lucky untill now
I'm planning on brewing some aya myself with Caapi and Chaliponga, so maybe that will work out in a total unexpected new way.
Anyway, you ask how to prepare for a purge. The first time I had to vomit was like the 6th time I drunk aya. I had been afraid a lot of vomiting before, so I was pretty optimistic that that seemed not very necessary for me. Curiously enough, when i drink the aya, it always feels pretty heavy on the stomach. After like 15 minutes, or 30 maybe, that just fades. Maybe some fermenting *burp* might come out, but after that it feels pretty good. The throwing up / nausea seems to start later on, when the trip is fully unfolding, and is, with me, always connected to inner experiences. That first time I'm talking about, a light hit me and i felt i wasnt able to conduct this much light; it just bumped on my stomach and the unblocking of this area made me vomit. The entities that were with me, explained that I had some nasty entity hiding inside me, and the only way to clean this was through vomiting. They gave me the following instruction: 'we can heal you, but we only can do that if you are consciously with us. You have to let these contents emerge in your conscious and don't block them. So, the light came, i made this vomiting reflex, and at the same moment these images came up from some traumatic childhood experiences. I knew I wasnt there anymore, but the moment seemed frozen in time and it was pretty intense work to let this emerge in my consciousness. Classic repressed unconscious material
At later moments, I always communicated with the entities present about what to do. Sometimes they advised: 'no no, don't vomit, just hold the feeling'. Sometimes they said: 'it would be the most easy way out if you vomited, but if you dont want, you dont have to, we can purge it the other way out, through your intestines, as well'. So I opted for the latter, but I regretted that afterwards, as it seemed to have taken me months to process these internal psychological contents through the intestines. The purging, in my experience, has always a psychological background, something that has to be released from the body. The vomiting is more a symbol of that. Of course it might be different for some people with really physical things (you know, like intestinal worms or what have you) that have to be purged, or with different brews. But for me, the entities always were a good guide on how to row the boat when nausea hit me.
So, wishing you the best with your first aya ! In the end, no one knows how to prepare, because it is always different. An open mind and heart seems the best recipe, along with some Light-beings as inner guide...