Squaring up your quilt blocks is one of the most important steps of making your quilt. It ensures that the surface of your work of art will be completely smooth and even, with no odd bulges or strangely long corners. Fortunately, when done right, squaring a quilting block is a simple process that will only take a few extra minutes. For best results you can square up each block as you finish it and avoid having to square up a large, heavy, nearly completed quilt.
Lay your cutting mat down on a large, flat space. This can be on a table or the floor -- wherever is more convenient for you.
Lay your quilt square down on the mat, then align the transparent ruler over it. This ruler should be square and have measurements of the same type (centimeters or inches) marked on at least two connecting corners. Align the ruler to the block by making sure the corners meet. The diagonal line on the ruler should help you align the ruler correctly.
Adjust the alignment by examining your square through the ruler. Make sure that it is the right length on every side, including the extra cloth necessary for the seam allowance. For example, if you want a 3 inch square to be exactly 3 inches when sewn, the square should measure 3 1/2 inches to allow for a 1/4 inch seam allowance on all sides. Move your ruler about as necessary so that the entire area of the square you need is covered by the ruler, including the 1/4 inch seam allowance on every side. If your square is already perfect there should be nothing to fix. If there is an issue, a bit of excess material should be visible outside the area covered by the ruler.
Take the rotary cutter and roll it along the edge of the ruler to remove the excess material. Once you have done this to one corner, flip the square around and repeat Step 3 to align the opposite corner. Once it is aligned, you can again remove the excess material.
Double check your work after you have finished cutting. Lay the square flat and place the ruler on each corner, to make sure that every corner is exactly right. If you have any problems, repeat Steps 3 and 4 until the square is completely even and perfectly square.