First and foremost, be accurate during the piecing process. Keep the 1/4 inch seams consistent. Inaccurate piecing will cause the quilt top to be off-square and often result in poor results during the quilting process. Each inaccurate seam compounds across the quilt.
It is also a good habit to press the seams as you construct the top and trim any excess threads.
Trim the backing so that it is square with respect to the length and width of grain. The backing is the first to be mounted on the frame and the top edge is assumed to be squared. If it is not, then the backing will end up at an angle. The best scenario is that backing is one piece. There is now a good selection of material up to 108" wide. If the backing is to be pieced, the preferred method is to join across the width.