I just lay the fabric out flat and weight down the corners with a can of food. You want it to be pulled out, but not stretched.
I measure with tape measure. I lay out triangles which makes it square. Like if you want a rectangle, measure out two corners along one side, then locate the third corner by measuring the diagonal distance from one corner and the side distance from the other corner,...
After I've located the corners, I use a straightedge to mark the edges. I have an 8 foot straightedge intended for woodworking