Attach a straight board to the miter gauge by running screws in from the back of the miter gauge into the board.
Loosen the miter gauge handle and square up the miter gauge using a square and one of the miter gauge slots in the table saw. Then retighten the handle.
Create a saw kerf in the board by running it thorough the table saw. Line up the cutting mark on the board you’re cutting with the edge of the saw kerf.