I designed to fit this cute little thrifted piano bench. It is long and skinny (12 1/2” x 34”), but I will tell you how to easily change the dimensions to fit a different sized bench too.
To sew a cushion approximately 12 1/2” x 34” (not including the straps, cut 40 squares 4” x 4”. Sew them together in a rectangle with 4 rows of 10 patches.