Originally Posted by sunnyhope
it doesnt get all straight; i know maybe a proper 4 by 4 square ruler would probably give better results
I sugest that you check You-tube and watch some of the clips on rotary cutting. No matter what size ruler you use, it is still important to line up your fabric within the 90-degree guidelines (either in the ruler itself or using the lines on the mat with the ruler)
I think you didn't align your fabric on a 90-degree line, and when you cut the next side, your angle was slightly off. Don't worry, it just takes a little practice.