Do you want your backing to be a bit larger than the top for quilting?
I would start with a sheet of graph paper, decide what your backing size will be, draw that on the graph paper- assigning the correct amount for each square ( like 72” long - 3” squares you would draw down 24 squares for example)
after having your outside measurements figured out and drawn on find the center then draw your center blocks.
if you don’t have any graph paper there are plenty of free graph paper sheets you can download and print off to use.