Firstly assess what the goal is. Fundraising, building a business, crowdfunding...?

There will surely be strong overlaps, but the objective should be clear before embarking upon the exercise.

One thing I would do, that I so often see missing or haphazardly guessed, is assess the TAM (Total Addressable Market). Is this about capturing most of a small market, a piece of an existing large market, or (best!) expanding the market size (the classic definition of disruption).

Even if this is a relatively small passion project, take the time to do create the plan. It can be your bastion to herald yourself and others to, but it can also be something you can share to achieve coherent feedback from experts.