Calm down guys. You're jumping on him faster than I jump on cows.
I wouldn't make your own diaper. If you were trying to make a cloth one, then it wouldn't be too difficult, but it would probably just be better to buy ones. Plus, you would need plastic pants, which aren't exactly easy to make. As for disposable, I don't even know how you would make those, and I'm guessing it would be hard to get supplies past your parents and then to actually make them.
If you truly want to make your own, then I would suggest to "look at the wiki". I'll actually give you a link to the wiki article on how to make diapers (cloth) It's here
Anyhow, I used to make fake diapers all the time when I was between the ages of...wow, maybe as young as 7 and up until 15. If you're not planning on wetting them it's fine. The wiki has all the info you need.
I used to make mine out of my mom's older bedsheets when I was a teenager. I never figured out how to make a plastic pant until a few years later. I'd take a plastic bag that would be big enough to fit over my waist with the diaper on, then I'd cut off the bottom corners at a 45 degree angle, so that the hole didn't have any "corners" in it, and it would slide right up my legs then, and if it was a little small, the hole that is, it would fit nice and tight around my legs, and I didn't need to worry about it leaking then.
Before I had any diaper pins, I would use tape, preferably either electrical or duct tape, to hold it together and on. Then I could make it as tight a fit as I wanted too.
But there are a number of items you can use to make a cloth diaper from. Bath towels work well for that too. I don't know how thick of a diaper you're after, but with cloth, you can make it however thick you want pretty easily. And the nice thing about a cloth one is that you can wash and reuse it. You just need to be careful how you do that if you are still living at home with your parents.
i used to make diapers with bath towels and a garbage bag. i would put the towel on with 2 pins, and then i would put the bag on like a diaper and tape it on with clear shipping tape, making sure it was really tight. i would wear them at night when i was like 12 or so. funny the things we would go through to satisfy an urge.......