Get the mat, turn it over and mark out how you want to cut it up. In half and quarters is ideal. Do all of this outside as it excess matting comes off the mat and makes things messy.
Carefully cut the rubber of the mat along the line you just marked out. Take two or three goes slicing slightly deeper each time, making sure your lines are straight.
Take your sections of wheat field and lay them with the rubber side up. Use PVA glue to run a fine line along one edge of the rubber. Push static grass against it to cover the black rubber. Do this with the opposite side and then let it dry for an hour or so. Once it's dry do the remaining two sides.
Your wheat field is now complete. Happy gaming.