First, calculate the total length of binding you need: (length+width) times two. Add about 12" (or more) to miter the corners and join the ends. If your wall hanging is 12" square, you would need (12+12)x2 12= 60 inches of binding. Now you calculate how big the square will be to cut the binding. If you are using 2 1/2" double fold binding, that is 180 square inches of binding. You need to start with a square 14 inches on each side; or 15 to feel safe. Iron in the diagonal. Place your ruler centered on the fold and start the first cut, then continue cutting.
Or do what I would do--cut five 2 1/2 crosswise strips from a full width of fabric, join the ends with a diagonal seam, and use straight binding. You will use less fabric and it's way easier.