How to keep a waistband rolled down?

Just put a belt on. Its probably the pants not you. If your tummy at the waist level is much larger than your hip measurement below your waist, then your pants will roll under your stomach like this. You could try wearing pants with a wider waistband that sits underneath the tummy so you won't have this problem. Or, you could sew a band of rubbery gripper material on the inside of the waistband. That might help.

Sew a band of non-slip fabric onto the outside of the waistband.

When you roll it down, it will hold itself to the skirt fabric

And LEARN to sew.

It's a life skill, like cooking, driving, doing laundry, and tying your shoes.