I did some searching off the net and got this from Nanook
To get pedro to dry effectively it must be chopped finely, or peeled and sliced. The skin prevents drying of the parts of the cacti where the greatest amount of alkaloid is located. It must be finely chopped (skin on) or the skin must be removed and it can be sliced horizontally or vertically.
Dry in the sun, in a drier, in an oven... Mescaline is chemically very similar to caffiene and it is stable at temps of 250*F (like in a low oven or a pressure cooker). Any hotter really and you lose it. Gone.
Once dried the cactus should be placed over desiccant for 24-48 hours, packaged and sealed.
If you are extracting... Why dry it? Add water to wet processed cacti, cook, and press out juice... Boil down to syrup, and extract that.
Hope it helps and good luck