Backing fabrics should have the same care requirements as the quilt top. It should be preshrunk if the quilt top fabrics were. Although muslin is an inexpensive option for a quilt back, keep in mind that it shows every quilting detail and does little to enhance the beauty of your finished quilt. Remember--you will see the back of the quilt when you’re using it. A coordinated backing offers more eye appeal.
Bed sheets for quilt backings aren’t recommended! Although it’s tempting to use a bed sheet because of its size, sheets don’t make good quilt backs. The thread count is much higher in sheeting than in common quilting fabric. The tighter weave causes the needle to break the sheet’s threads when it pierces through, rather than pushing between the threads as it does with quilting fabric. It leaves holes and diminishes the stability of the sheet.
To piece your quilt backing, use 1/2" seam allowances for added stability. Sew the pieces together along a pair of long edges. Press the seam allowances open.