Sometimes your backing fabric is just not wide enough to do the job! Here is how you can use a diagonal seam to achieve a quilt back from the length of fabric you have on hand. Tip provided by Eddie Walker Quilts
Start with your length of fabric . . . Fold fabric diagonally and cut along the fold. You may wish to press this fold so it is easier to cut using scissors or a rotary cutter. You will create 2 triangular pieces of fabric
Slide one triangular piece down until the desired width and length of backing is attained.
Note: Measure top to bottom and side to side through centers to verify backing will be the desired size. Trim off ears and sew pieces together using a 1/2″ seam.
Limitation: backing size is limited to 150% of backing fabric width. For example, if the backing fabric is 42″ wide you could create up to a 63″ backing.