How do you get the band tight enough that it doesn’t fall when your wrist is down and loose enough that it doesn’t constrict blood flow?

I’ve tried going tighter, but it started to hurt my arm. I’ve tried going looser, but it fell to my wrist when standing/walking that it kept bumping onto my bone, making my wrist hurt. How to strike a perfect balance? It’s been months of trial and error and I can’t find the right fit.