Advice for substitute teacher

So I’m asking on behalf of a friend who recently became a substitute teacher, she’s fairly young (early 20s) and I think subs for elementary and or middle school. She’s having issues getting the classroom under control, she tries to raise her voice to an extent but the kids either outright don’t listen or will go quite for a bit then go back to being rambunctious. Is there any advice you would give her to help keep classrooms under control? Thank you in advance!