Completely cover the window opening with heavy paper and tape into place.
Mark the room side of the paper with the word "face".
Using a sharp blade or knife, cut the paper as follows:
Inside mount blind:
Cut the paper along the exact inside opening of the window.
Outside mount blind:
Cut the paper the exact size you want the blind to be. (If you want the blind to sit out over the window by 1", cut the template 1" to the outside of the opening.)
Draw a diagram of the window to be covered on the face of the template, indicating top and bottom, and showing (to the nearest 1/8) the exact inside measurements of the window.
Roll or fold the template and ship in an envelope or tube to the attention of your contact at the company.