The shape
Stand in a corner and look around.
Four straight walls and four square corners, or is there a nook, a chimney, or a bit that juts in?
The walls
How long is each wall?
Measure along the floor, baseboard to baseboard, corner to corner. If furniture is in the way, measure it in pieces and add them up.
Doors and windows
What is cut into those walls?
Measure the hole, not the wooden trim around it. The distance is from the nearer corner of that wall to the edge of the hole. Tap a wall in the plan to fill in the wall number for you.
Height
How far is it from the floor to the ceiling?
One measurement is enough. Pick a spot where the wall goes straight up.
Take it with you