There are a lot of ways to do this, but here's one. The first thing I'd do would be to mark the walls where you want the table top to set with a pencil line or masking tape. (I'd use a level to make sure the lines are all at the same level, rather than measuring up from the floor.)
The next thing isn't too hard - measure the length of the back edge. Taking an inside measurement with a tape measure is a little sketchy. The right tool for this job is pinch rods:
But you can probably improvise some other method to get an accurate measurement.
The next step is to measure the angles of the back corners. You could use a bevel gauge
to measure and replicate the inside angles at the back wall, and transfer those angles to mark your tabletop material. However a bevel gauge is too small to make a really accurate measurement of a long line like this. But it's easy to use and inexpensive, and will get you close. Just don't trust those measurements without checking them.
Before you cut, check the diagonals. If the diagonals on your marked up piece match the diagonal measurements of the space, you're on the money; if they don't, you're off. Again making an accurate inside measurement is best done with pinch rods but you can improvise something that will work.
If you want to be extra careful, you could mark up and cut out a pattern on cardboard first. With a pattern, you can check fit before you cut - get it just right, then carefully transfer the cut lines from the pattern to the material. If you're patient and / or clever, and have a lot of cardboard, you can make a pattern by trial and error without taking a single measurement.