Before designing any behavior plan, collect concrete data on when and where the behavior happens, how long it lasts, and what triggers and follows it. Even if you think you already know the problem, the numbers and patterns tell you what actually works to fix it.
If you're creating a behavior plan from scratch, the very first thing you're gonna do is take data. I know that's not exciting. You're ready to get in there. You're ready to help provide supports, but you have to figure out what's going on. Even if you feel like you know, you're like, "Oh, I know this is an escape behavior," you want to take some data so you can get a better idea of what settings this is happening, how long this is happening, what are the common things that happen before and after. So data is the first piece in every behavior plan.