Two kittens adopted us several weeks ago. They are female and one is black (I named her "Bella" after my Dracula bella orchid), and the other is grey (my cousin, Linda named her "Buttercup" before I could stop her). This is their story. (No, not Linda's!)
Step 1: Tools & materials you'll need.
3. Marker (felt pen, etc.) if you want to mark the cloth before cutting.
4. Four safety pins.
NOTE: This is for a medium-sized cat. You will need a bigger piece of cloth and more safety pins for larger animals.
CLOTH NOTE: I have found that tee shirt material works best (not sure, but I think it's called "Jersey"). I did try another type of cloth that was more "crisp", but it failed miserably in less than a day -- it frayed and ripped, and of course the poor cat was desperately trying to lick the frayed threads. Be careful what material you use.
FASTENER NOTE: You can of course use something fancier than safety pins, but before you sew on buttons, snaps or velcro, remember that the jacket only has to last a few days - until the stitches are removed. And while it's being worn the cloth will stretch and have to be repositioned, you should look under the jacket every day to check the wound and if the jacket rips, gets soiled or has to be re-tailored you may be sorry you didn't just use quick & easy safety pins.
MOST IMPORTANT NOTE: The whole purpose of the jacket is to protect the wound and stitches from being licked or chewed. It's okay if your cat licks the cloth; no harm done. BUT IF YOUR ANIMAL BITES OR CHEWS THE WOUND AREA THROUGH THE CLOTH then you must put the head cone back on. Or possibly use a heavier cloth, or a patch of chew-proof material sewn over the wound area. So far, my cats haven't made any effort to chew through the cloth - only lick it - so hopefully you'll have the same luck.
PICTURE: Buttercup gnawing on the hated cone.