hello,, documentary of any kind of subject do the following...
>>>do some research.....
even if you know your topic well still do the research, you can use internet and library to find the the information, also talk to people who know about or interested in your subject
>>>interview people....
get interview from people who have knowledge in your topic
>>>outline your documentary...
organize all the parts in an order that is interested but make sure it will make sense
>>>keep a diary....
keep a diary where you write, how it went, mistakes you made and how you avoid them it can be a help next time
>>do a screening...
show it to someone like friends ,parents to give critique, then see if there is something to fix