Documentary work is measured in months rather than sessions, so the preparation that counts is about access, consent and the shape of the story — not what's in the bag.
Access is the project. Who lets you in, who introduces you to everyone else, how often you can come back and for how long — settled in plain terms at the start, and revisited as things change, because an arrangement nobody spelled out tends to quietly end at the point the work was starting to get interesting. It's worth being equally plain about what you're not asking for, so the people involved aren't guessing.
Consent runs alongside it and isn't a form. People should understand what the work is for, where it might appear, and what they can ask you to leave out — and that understanding needs refreshing over a long project rather than collected once at the door. The same honesty applies to the edit: a frame can be entirely true and still misrepresent an afternoon, and deciding in advance where your line sits is easier than deciding it while looking at a strong photograph you'd rather keep.
Then the shape. A written premise — a sentence or two on what this is actually about — plus the scenes the story can't be told without gives a long project a spine, and keeps months of shooting from becoming an archive nobody can sequence. Captioning names, places and dates while you still remember them, and backing up as you go rather than at the end, is what makes that archive usable; a documentary project is judged as a body of work, and the body is built by selection.
A few things to plan for