Since you already have the gear, know the topic and the person it's based on, the next step is to write out the basic storyline, or "outline", you want the doco to follow, including main points you want to cover, people you want to interview, locations, and any "aha moment" ideas you might have.
You want to plan out who's going to assist you, if anyone, and see what their availability is.
Based on that hard work you'll want to budget everything out, even if it's $300, just put it on paper so it's not just a floating number in your head.
After that's done you'll want to storyboard your shots, even if it's just writing them down shot for shot. Then schedule your shoots, even if it's just an opening shot. Be sure to keep in mind you really want some WOW moments if possible and dynamic scenarios, like interviewing someone in a provocative setting ( i.e. Alzheimers sufferer interviewed at the grade school, or college they went to )
That should get you started
