First off, I wouldn't recommend doing this if you're actually going to be traveling in them. It's going to compromise your ability to layer comfortably and make 'em less durable than the original. There's a reason most people just leave em alone. But, I did do this last year with an old pair, seeing as I'm pretty well settled. What I did was turned them inside out, put them on and saftey pinned the way I wanted the legs and crotch to fit. Then I sewed them along the pins with a basic stitch flipped em back right side in and made sure they fit (this way you didn't waste too much time and the original seam is still there if they don't fit right) then sewed another basic stitch over the first one to secure them. Then I cut the excess fabric seam out and did a finish stitch along that. Keep in mind that these are made of rigid fabric, not stretch like most tight pants, so don't make them too tight. And only take the crotch in a little bit, no matter what. Otherwist yer gonna have a pair of low ass huggers. Good luck.