These were my old jeans, and I didn't want to throw them away because the fabric was so deliciously stretchy and I love how jeans have contrast stitching. This pair in particular was worn in and cozy, and I decided to cut them into pull-on jeans for my older son.
To make them, I cut them to height using a pair of sweats that fit him as my pattern. I added an extra inch for the waistband. Then I curved them in, left the width as it was, and stitched up the center seam.
I originally just turned the waistband, wanting to use the stretch of the fabric to hold the pants up. They immediately fell down (yes, even though I measured them).
So I cut that off and added a 2" piece with elastic inside. The rise is a bit short in back. Next time I will leave closer to 3" to play with as I do the sizing and fitting around the waist.