Now that you have the film all lined up and smoothed down so that it's not moving, I hope you didn't have soap in the water ;), use a thin hard card and tuck the edges of the paper into the sides of the window where it disappears behind the guide. Tuck just enough film in there to hide the edge. Too much and it's going to be tougher to install. Once it's all tucked in nicely, draw a line where the fold is in the tuck with a very sharp pencil. This will give you a sharp line to use while cutting later. If you use a dull pencil, you will get a dull line.