I assume the factory edge is straight.. so that leaves me with 3 uneven sides to cut. Those being uneven is going to be even more pronounced when I make my hem... and even MORE pronounced when the hem is rolled. Maybe I am being too anal.. but I want it to be right, not some skewed crooked POS!
The large area and carpenter's square is best but if you do not have a big area to work in try folding into quarters and using the two folded sides as the straight edges to square with. This works best with one of the rolling cutting wheels. You can use the project board to square your folded sides then run a straight edge across the line you want to cut on the project board mentioned above. You can reduce a hammock sized material down to the size of the cutting board this way.