First of all, you're going to want large Tshirt. I'm going to use this blue one to line a pair of pants I'm making out of a large white Tshirt.

First, cut off the sleeves.

Then cut off the collar. Sounds familiar, doesn't it?

You're going to want to cut up the SIDES, not the middle of the shirt. Fold in half [wrong sides out] and sew until you get to the sleeve. This will be your crotch.

Do this to both sides of the shirt. The logo should be wrapped around the side of your leg, not cut in half.
This is both my legs, one inside out. I know the blue shirt is a little short, but that's fine.

Pin up the crotch and sew.

Now you're going to want to make a waist band. I just used fabric from the blue long sleeved shirt.
Sew on.

I made my waist band twice as tall in the back than in the front.

Now your pretty image isn't cut in two!