My suggestion whenever asked is to first make a plan to scale. Graph paper and a pencil is all one needs along with a tape to measure. Locate fixed items. Then your walls. I used masking tape placed on the floor to get a feel for spaces.

If staining the floor is your plan, yes I'd do that first after locating any plumbing that may require cutting into the concrete.

I'd put XPS against the foundation walls, then frame them. Then electricity before drywall or paneling. Consider lighting and a mix of cannisters, sconces, pendants and/or outlets for table lights. If you're planning a drywall ceiling do it before the walls or drop ceiling following the walls.

Here is a link to photos of ours. Maybe some ideas. We only finished about 700 sq. ft of approximatelyt 1,000. Good luck.

Here is a link that might be useful: Basement