CB
The way I do it is; I overlap the edges about a 1/4" and use my trim seal iron set on high with the flat shoe installed.

You can use a regular iron set a little hotter than normal and just use the tip. You don't want to shrink the film just stick the two pieces together so just work the seam. The film don't burn as easy when smoothed out on the glass.

Make all your seams face the rear and towards the fuse, and put dark colors on top of light colors.

When I shrink the film I use a heat gun and go easy on and around the seams especially if they are over open bays.

DEG