Put on the pants and pull the waist back to your desired width.
Use chalk to mark where the jeans fit.
Rip the seam in the part of the jeans that you marked and cut the waistband
The jeans should have a cut in them here.
Pin the jeans where you marked the chalk with the fold on the inside of the jeans.
Sew a single straight stitch on the outside of the jeans on top of the fold you created.
Then you can cut off the excess fabric.