You can start by getting out your essential plant dissection tools. OK, you can start by going out and getting some essential plant dissection tools. If you skip this step, you suck. First you need something to cut fine sections of cactus skin with. A box cutter is good, even just razors are pretty good.
Scalpels are good to have around.
Of course, you can skip all that and use a big knife and your hands to do everything, but it isn't as much fun. Lay your cactus out on a flat surface that is mostly clean. This surface will soon become a slimy, stinky, ugly mess. First thing you'll want to do is remove all the spines. There are two ways to accomplish this.