Looking at the Erowid information on mescaline content of San Pedro, you could be looking at anywhere between 160mg to a gram. Although most people who are doing extractions and posting results on forums like these are generally quoting yields of around 1% full alkaloid, which means that you would get around 500mg full spectrum alkaloids. I wouldn't bother with an A/B extraction for such a small one-time dose.
I would make a tea out of this amount. There are different teks for different amount of boiling times, using dried or fresh, how far you reduce it, how you cut the cactus, extra ingredients etc. It's basically just boiling the cactus (dried or fresh, doesn't really matter), straining it out, reducing the tea down to a drinkable quantity, like a shot-glass full. You can go further and make a resin which you can eat very easily.
If you can stomach it, then I suppose you could put the powdered San Pedro in to gel caps and eat them, but I'm not really too fond of that idea.