All right, to begin sewing virtually any garment, what do you need? A pattern. Seeing as how I'd never made pants before, and I have a big-ish and hard-to-fit behind, I felt that tackling the alteration process on a standard pattern from scratch would be a little out of my range. I didn't really know quite how to start, until I remembered what was in the back of my closet! Do you have an old pair of jeans that, for whatever reason, you don't wear anymore, and yet they still fit fairly well? Great! That's what I used. (If not, the local thrift store is a fantastic place to find such a thing). I figured that this would be a great thing to use, because it'd be a pattern that should already fit fine!
There is, however, a down side. In order to get these old jeans to turn into a new pattern, alot of tedious work is required... tedious work in which a seam ripper is your new best friend. Yes, the dissasembly process. Do this in the car, while waiting at the BMV, and virtually anywhere that you can. Do this, and it will go faster than you think! (It also helps that it's a pretty interesting process for those who like to see how things work!).
Dissasembly is not only a means to acquire your pattern, but it is also a very importang learning process. Take notes! Taking these jeans apart seam by seam will help you envision how to put them back together again, so write these things down! The more you learn about how pants work, the easier it will be to make a new pair! (Understand: nothing is unimportant. Learn even how the seams work!)
See you at the next step!
NOTE: I wouldn't suggest using tight fitting or 'stretchy' jeans-- the jeans have probably molded to your body shape, and will produce patterns that will not provide the same result. If you really insist, I would increase the size slightly during patternmaking.