The first step in Designing a Behavior Tree is to pin down the Expected Behavior we would like to see in our character. This seems like a simple phase, but trying to take all the cases into consideration isn't trivial. However, it will avoid many headaches later on.
While writing down the Expected Behavior, try to be as specific as possible. If something is unclear, try to rephrase it. The sentences that you use should be short and always add information. Don't be scared if you have sentences starting with "In this case…" or "If…", because it will just mean that you are taking all the different possibilities into consideration. Once you have written it, read it out loud, maybe to a friend, and ask him/her if he/she clearly understands its meaning.
This is my attempt of describing the behavior we are going to implement...