The jeans must fit you well when zipped up all the way, even if you cant button them. Carefully rip off the entire waistband and zipper, leaving the part of the jeans that used to hide the zipper.
Cut the top 1/3 (somewhere around where your ribcage would be) off of a t-shirt or tank top, discard. The t-shirt must fit you snugly or the material won't hold up the pants properly later!
Pin the cut end of the shirt to the waist of the jeans, both with right sides out. You'll have to "stretch, hold, pin" a lot!
Stitch a straight line all the way around the waist about 1/4-5/8 inches from the edge, then go around the edge with a zig zag to prevent fraying.
When you pull the t-shirt up over your belly,, the inside (wrong side) should be showing. Fold it down and it will look like another layer peaking out! :)