Jeans are hard to take up, especially in the waist, due to the massive layers you'd be trying to sew through. My sister tried and tried. What she finally did was to take heavy black elastic and stitch it to the inside of the waist band and she cut it an inch or so shorter. Then she pinned it to the other end of the waist band, in the middle, then increments of a couple of inches.
Stretched out so when sewn it would draw the fabric in.
She'd sew a single seam down the middle using a big zig zag stitch with heavier thread and usually a clear thread so it wouldn't show. Then she'd go back to the same end she started on and do one edge then go back and do the other edge.
So she made her own elastic waist for her daughter's jeans.
When he daughter wore her jeans with a shirt tucked in she'd wear a belt with them and no one could see the gathered looking waist band. Tops out they didn't show at all.