Squaring Up Your Backing
Fold the backing in half on your seam with the selvages edges together at the other end. Smooth it out so that the fold is nice and flat and there are no wrinkles. Standing by the fold side, fold it again from the right to the left. You know have a fold at the seam and a fold on your right. There are 4 layers of material now. Pinch the fabric at the second fold to make sure all 4 layers are in the fold and there are no lumps or wrinkles.
Another way to make sure all 4 layers are in the fold is to take a ruler and run along the inside of the fold...be sure and be gentle so as not to distort the layers.
If your backing is extra large, continue to fold until it is small enough and will fit on your cutting mat.
Be sure to smooth out all the wrinkles and pinch the fold each time you make a new fold; just to be sure all layers of fabric are accounted for.
Go to the opposite end (uneven cut edges or non-selvedge ends) of your folded backing and using your ruler and rotary cutter, make a nice straight cut to even up the edges. Continue to cut the width of the quilting fabric on the other end if not folded twice.
Turn your fabric so the longest part runs parallel to the longest part of your mat. We now have all three edges straight and lined up on the mat (the seam fold, the two uneven cut edges).
On the other selvedge edge, check all four layers and trim to even the backing fabric with the shortest edge.
After squaring up your backing, measure it again to make sure that it is a total of 6-8 inches larger than the top, if you are going to have it long arm quilted.
A machine quilter will require a minimum of 4 inches of excess fabric on all sides. If you are using a professional machine quilter, often 8 inches will be needed. Guess what? If you cut too short, you must add it back on in order to be able to put the backing on the quilting frame. So always ask before cutting.