When Gracie got her first pair of pants with an adjustable waistband I spent a lot of time examining them to figure out how they were put together. After all, if I could figure out how it was done, then I could replicate it in homemade clothing and even convert regular waistbands that were too big without too much effort. If you're dealing with RTW clothing, you'll need to rip out the topstitching holding the bottom of the back middle belt loop to the waistband, and rip out one to one and a half inches of waistband on either side of the side seams.
For RTW, simply fuse the interfacing and open the waistband for marking.
If it's an old pair of RTW pants that already has an elastic waistband, I'm afraid this can't be done without ripping out the old elastic, but it will work on just about any pair of jeans that didn't come with one--even adult sizes! (Perfect solution if you're in-between sizes)