Not out of the norm. I know tons of people who have done it themselves with great results!
I tried it, and I had no idea what I was doing. Turned out OK at best. I've seen others with better results.
The way to do it, and I'd only suggest it if you know what you are doing, especially on good pants, is to:
1) Turn the pants inside out and put them on.
2) Grab the amount of fabric you wish to sew on the inside thigh, I'd recommend not going as tight as you like initially, because you can always make them tighter, not looser.
3) Take sewing pins and run them up the side of the pant. I'd recommend pinning in up from the knee to just slightly below the crotch, as mentioned earlier, on the INSIDE seam. Outside will look like shit.
4) Sew by hand or with a machine. For best results, use a machine. I'd recommend going over the area 2-3 times to ensure a solid seam. You don't want to blow it out. I'd also recommend a nice sturdy nylon thread if you can find it.
5) *Optional* cut away the left over fabric. I would only recommend this is it creates a weird bulge in the thigh area. However, if you do it right, it shouldn't be tight enough to create a bulge. Cutting the material will also decrease water-proofness in that area.
Good luck! I'd recommend starting with an older pair of pants because, chances are, you will make a mistake.
Also keep in mind, your new seam will not be water proof unless you you have it "taped."
And also I'm not responsible if you fuck up a good pair of pants <3
