Begin with initial discovery and experimentation, get a feel for what you’re trying to do, read documentation, build prototypes. Then start putting together a plan.
Start with a one page document (not counting references/resources). Doesn’t have to be a page, but the longer it gets, the less folks read.
This document should outline three key details.
What is the Problem that needs to be solved? Who needs it solved?
What is the Solution to the Problem? How do you measure that it was solved?
What are the associated Risks? How do they constrain the available solutions?
The client could be internal or external, the result is the same. Establish clear expectations. More often than not, this is one of the hardest parts.
I said three and we have six. See, expectations can change, aren’t we supposed to be Agile?
That is not a bad thing, that is real life. Adjust them accordingly, but establish expectations. They’re your guideposts.