If more is available brew up the whole thing. You should get results from that much cactus.
Conventional wisdom says most of the mescaline is in the darker green flash, just under the waxy outer layer of skin. Most people discard the white core, but there's alks there too, so these days I leave the core in.
You might want to peel off the waxy top layer of skin. This will involve freezing and thawing it a couple of times to loosen the skin and then scrape the shrimp shell-like skin away from the green flesh. The freeze/thaw also helps break down cell walls.
Take the resulting slimey cactus mess and liquify it in a blender with about 8 - 10 ounces lemon juice and a little water. I'm lazy so I just buy a big bottle of the real lemon juice from Whole Foods. Take the fresh green lemony cactus snot and put it into your largest (at least 2 gallon) cooking pot and cook it, stirring pretty much constantly for at least an hour. The foamy mess will take the first 20 minutes or so to cook down into a manageable liquid, at which time I add a little more lemon juice. Let cool.
Now go cut up an expendable t-shirt so you have a decent sized square of fabric. Take a second large pot and spread the fabric across the top. Slowly pour your soupy mixture through the t-shirt and then gather up the sides of the shirt to envelope the pulpy mass. Twist it up at the top and squeeze out all of the juice into the pot.
Pour the resulting tea liquid into a large small mouthed soda bottle. Buy a bottle of club soda. But Tums antacid tablets. Depending how far you cooked it down you'll have 500 ml to a liter of tea. -CHUG IT ALL - and chase it with the Club Soda. In an hour or so the nausea will start. When you think you might puke chew a couple of antacid tablets and chug some water. When you really know you're gonna spew munch a couple more antacid tablets to coat your teeth and esophagus.
Now hang on and enjoy your Big Green Spewey Adventure. Hope it does the trick but if you don't get high try some Bridgesii next time.
My flesh moves, like liquid. My mind is cut loose.