lol i don't think you want to boil it... the best way to do it is to:
use a 12-14 inch cutting thats 3 or 4 inches thick. keep it in a dark box for 72 hours before using.
1

eel all of the green plastic like skin
2:de-spine, by cutting in a 'V' shaped cut to remove all spines and spine "roots"
3:cut into 1/4's the long way
4: remove the white spoungey core and throw that out (it contains very little mesc if any)
5:blend the remaining "sticks" of cactus, for a very long time until it is liquid, only blend 1/2 of a stick as it will make alot of foam (and make sure to keep the foam)...
6

our all of this nasty ass goup and foam into some container. that will hold that much shit
7:take a bucket (like a 3 gallon would work the best), wash it out because it is gonna have you yummy drink in it, and then strain all that goup through a T shirt, and make damn sure you squeeze it very tightly until you end up with balls of paste. throw out that nasty paste...it has mesc still in it but trust me you have enough and eating that shit is not hardly possible.
8: pour this thin/thickish sludge into a cup, you should prolly have about a pint to a quart of this shit and you could mix some shit with it...but it doesn't really help, and it just adds more to drink.
so just slam that bitter sour fucking nasty ass shit as fast as possible (don't throw it up or all this work will be for naught).
you will not feel good for like 15-20 minutes but after that the taste and stomach ach goes away , then a hour or hour and a half... well you get it.
U WIL B FUCT ^
i love to do this once a year or so, it's a unique experience.
-Cult