I found a pattern online to make Q Snap covers but now the link is gone. It took me a bit but I found the measurements that I wrote down after doing the math for different sized frames. I'll try to explain.....
For a square frame -
4 X inches plus 1 by 7" wide for fabric and 2 X 2 sides plus 1" for elastic (elastic should be 1/4" or 1/2" wide)
8 X 8 frame 4 X 8 = 32 plus 1= 33" long by 7" wide for fabric and 16 plus 1= 17 for elastic (you need two pieces this length)
8 X 10 frame 2 X 8 plus 2 X 10 plus 1 = 37 long by 7 " wide for fabric and 18 plus 1 = 19 for elastic (you need two pieces this length)
cut the fabric then fold each long end in 1/4" and iron down, then fold each long end in 1/4 and iron down. This creates a sort of pocket for your elastic. You will then sew down the inside fold to close the pocket leaving openings at the ends to thread the elastic. I then put a safety pin on one end of a piece of elastic and fed it into the pocket. when the end with out the pin was even with the end I inserted the elastic I pinned it so it wouldn't feed through (since it is sorter than the fabric) this causes it to gather. Once on the other end pin that to the edge. After both pieces are fed and pinned you will take the fabric and join the two short ends together with the right side of the fabric facing itself. Sew a 1" seam. I wrote the frame size on the seam in sharpie marker for easy reference. Note: The elastic I bought did not play nicely with my sewing machine so I ended up hand sewing the elastic pieces down and just machine sewing the seam. If you don't have fabric in the lengths you need to sew two long pieces together to get the length, just remember to leave seam allowance.