How do i become friends again with someone i haven't talked to in a while?

I'm only 16 but as i've gone through secondary school I've changed through different groups of friends. I want to become friends with some of the people from the old groups but because i'm not in their classes, it's hard to do. I also feel its kind of awkward when i try to talk to them, but i really miss some of them and want to become friends again. Can anyone give me advice please?How do i become friends again with someone i haven't talked to in a while?
Once u get away from a group of friends, it's hard to get back really close to them again. However, take slow steps. And what really helps is making a sort of inside joke with them. Then, whe your with other friends, they will think u guys are friends. Sooner or l8tr, u guys will be bff's. Also, try to make friends with them one at a time.