I've never grown them, but I'm awesome at extracting mescaline.
When you cut the cactus cut like 1cm thick slices (cut horizontally not vertically) then cut the hard core out of them. Dry the remaining part in the oven at 225°F/110°C for an hour or two. Grind up the dried material. In a large beaker or glass or stainless steel pot mix the dried powder into enough water to cover the cactus powder and have an inch of water on top of it. Add acid to pH ~3. Then add nonpolar solvent (use about a quarter of the volume of water used. Mix vigorously and thoroughly. Then separate (a real sep funnel is cheap and legal to buy and your beat friend when you do extractions) and do another wash with nonpolar. Throw out these nonpolar washes as they only have fats, oils, waxes and other stuff that will just be a pain in the ass down the line. Filter the aqueous layer and set aside the cacti for a second pull. Now raise the pH to between 11.5 and 12. Set aside about half your water's volume of nonpolar solvent. Use half of it for the first pull and mix it thoroughly. Separate and collect the organic phase. Use the remainder of the organic phase to do three washes of the aqueous phase to pull out any remaining mesc. Pool the organics and evaporate about half the solvent. I like to wash this 3x with 25mL brine. At this point you have a solution of mescaline freebase and it's time to collect it. Salting it out with an acid is the way to go (evaporating the solvent and collecting the freebase gives you a less pure product). I use dry HCl to salt stuff out, but I wouldn't recommend it to anyone who needs a recommendation for how to salt out an alkaloid. There's a bunch of teks for salting out mescaline with wet mineral acids, acetic acid, citric acid, and probably others, but that's plenty of choices. I always recrystalize after salting because I want the highest purity I can get. The last batch I could test was like 99.3% according to the GC.
There's several good nonpolar solvents available OTC for a mescaline extraction. Toluene and xylene are both good and available at Home Depot, d-limonene is also good and available at the grocery store in foodsafe grade. I'm partial to ether for many extractions and lots of other things but it's fucking dangerous if you don't know how to use it properly and it's tough to get OTC without having to distill it (please don't try this at home).