Step 1: Checking a square
The dark brown piece of wood in the photo is the end section of a piece of veneer plywood I have been carrying around for several decades. The white paper arrow points to the factory cut edge of the sheet. This makes a trustworthy straightedge. The white chipboard piece of shelving raises my work surface for this Instructable to the same height as the plywood straightedge.
Step 2: Lay down a piece of paper and tape it.
Step 3: Score a line.
Step 4: Flip the square
Step 5: These lines are parallel
But, earlier I said this square was not square when it was new. I made it square.