A brief can be many things, but it's easiest to concentrate on a short summary of features during early discussions.
Agencies, creatives, technicians and partners, will be looking for a simple brief to get a conversation started. This communicates what it is you want to make.
We recommend you start by writing a simple feature list for your new commission. Keep it short and prioritize needs over wants but do leave a little room at the end for some fun stuff.
Never try and solve a technical or creative problem yourself as part of your brief. It's the equivalent of inviting a plumber to your house and directing them to use materials, techniques, timescales and ideas of your own devising. State the problem you wish to solve and let them plumb.
Try limiting your feature list to 1 page. It should have 4 headings:
This simple process will have the benefit of forcing you to really evaluate your needs resulting in a short and digestible read for your potential partners.