I am not sure what you mean by "squared up to the top" Are you talking about making sure that fabric grain is straight for each piece in the backing? Are you asking how to align the various fabrics so they are even on the back visually? Are you cutting the backing the same size as the top? my answers is for the first, you just make sure when you trim the pieces for piecing the back that you make sure you know where the length wise straight of grain is which can be determined by gently pulling the fabric scrap in different directions until you find the one with the least amount of stretch. That is the lengthwise straight of grain in most cases. As for the second question...if it is visually, that is very hard to do on a backing. If I piece various scrap fabrics together to make a backing, I generally purposely offset them a bit so it looks intentional. As you quilt, the backing material does shift a little as the batting takes up space and it is just very difficult to keep everything lined up perfectly on the backing as you can not see it and generally are trying to keep the top lined up. As for the third, I make my backings about 3-4 inches wider on all sides to make sure I have enough backing all around. Most longarm quilters will require your backing to be 6-8 inches wider and longer than your top.