Lets measure your front take a note of that figure, Mine was 18 1/2 inches. For the back I like to add 8 inches to that figure so I came to 26 1/2 inches x 18 1/2 inches. ( quilting makes a big difference to the block size ) so go a head and cut a piece for the back and then cut that in half. I have two pieces 18 x 13 1/4 inches.
Now we need to hem those pieces width ways. Measure 1/2 inch from the top fold wrong side of fabric to wrong side of fabric and press and then re fold that 1/2 to conceal the raw edge. Pin in place.
Top stitch that hem both the top and bottom edge 1/8 on an inch in from the edge.
Now lay your two pieces on your cutting mat right sides down, Line them up so that they measure the size of your cushion front. Your pieces should overlap.
Pop your cushion front on top rights side facing up and pin in place.
Finally sew around your cushion with a 1/8 seam to secure all your pieces together.
Now all thats left to do is make and attach our binding and we will do that next time. Have fun.