maybe some people remove some parts but all the times I did the cactus resin, I made no preparation at all, I just cut up the cactus in small pieces with all parts included (spines, core, everthing), and simmered maybe 4-5x for a couple of hours each, with half a lemon squeezed each time. It works perfectly, I had no nausea at all, it was nicely potent (as described below).
Just so you can more or less compare to whatever results you have (and let us know after!), once I used a 1,7kg 50cm san pedro cutting, and that yielded 65g of resin, where 5g was a nice empathic dose (perfect for watching a concert for example, very good to connect to others socially), 10g was an average nice psychedelic dose.
The way I did to consume was to make it in small pellets (0.5g each), roll in flour and swallow it like a pill followed by some water. This way its not hard on the stomach and you feel no taste. It hits in about 2:30 hours. If the pellets are bigger, its not as effective, takes longer to hit, is harder on the stomach.
good luck