Mostly it takes careful measuring.
I just began measuring in the top left corner and planned the first color down and to the right to be the same length. The actual corner is a separate piece when it comes to measuring. Then the next color down and right the same length, etc. Carefully cut the 45 degree angles to join the pieces so that they are going the right direction! (Goofed once and wasted some material.) Don't forget to allow an extra half inch on each piece for seams. When you get near (or at) the top right corner, stop, then move to the bottom right corner and work left and up. The final 2 corners (top right and bottom left) will be whatever is leftover. I managed to make the bottom left corner work out to a perfect 45 degree angle where 2 colors joined.
After I had it all laid out around the quilt, I sewed each side together and attached to the quilt.