I am often too impatient for the fussiness of finding and using a pattern to sew clothing for myself (and I often find myself deciding at 10 p.m. that I Absolutely Need Something New to Wear Tomorrow!), so I made my own basic shirt pattern.
Homemade shirt pattern pieces.... Don't think I'll be working for Simplicity anytime soon.
I didn't use any technical procedure (in fact, I didn't know there was an actual science to it until just now). I took a simple top that fit me well, and traced the basic components onto tissue paper, keeping in mind the places where they would connect and ensuring they matched up. I ended up with front and back pieces (cut one each, on the fold), and a long sleeve (cut two, on the fold) as well as a short-sleeve option (I just traced the long sleeve and made it shorter).
For extra stability, I laid pieces of clear packing tape over the tissue paper; the only drawback is that the pins tend to get sticky from the tape. (Next time I would use a heavier paper and skip that step.) The resulting pattern pieces are not pretty, but they've held up through quite a few uses – and I can whip up a new top anytime I fancy.
Next I'd like to create a basic bodice block (and the companion sleeve). It's a custom-made template that you can simply alter, by adding darts and changing the neckline, for example, to create any number of shirts, jackets and dresses. Drafting your own patterns requires a lot of trial and error, but it's so rewarding to be able to say, "Yes, I made the shirt I'm wearing (so please don't look too closely at the hem)." Have you had any luck creating your own designs?
