How do i HAND sew an elastic to pants?

Okay, so i was stupid. I thought i knew how to hand sew an elastic to these sweatpants. I was trying to fix them because they were too big. Now i don't know how to put the elastic on . I'd appreciate a video or a clear explanation. I'm kinda new to this. Basically , i want to know how i can hand sew an elastic to sweatpants.