Apply the 3/4″ foam tape all the way around the outside edge of the winsert. This will compress slightly when the winsert is installed and will make a good fit within the window frame.
Use your fingers as a guide to align the tape with the frame. Pull the backing strip clear as you apply the tape. Try not to pull the tape too tight around the corners (as in the photo). Instead, bunch it up slightly on both sides of the corner, to create a bit more fullness that will fill up the corner of the window frame instead of leaving a little gap. Press down on the tape when you’re done sticking it to the frame.
There are other possible ways to fit the winsert into the window frame. For one alternative edging, visit this EFI page.