The most common type of pan flashing is nonflexible flashing tape. You use this stuff all over the jobsite, but it poses some challenges in rough openings. Corners are the tricky spots. Regular peel-and-stick roofing membrane bends, but it isn’t flexible in three dimensions. You usually need to cut the corners and make patches. Some manufacturers offer premade flexible patches, but you can do it with scrap membrane, too.
The first thing I’m going to do is remove just part of the release sheet off the back. It makes it a lot easier to install than removing the entire release sheet. Some of the manufacturers put a little cord inside of the tape itself so that you can separate the release sheet. I’ll just dig that out here; it’s a little wire. And when you pull that wire, it’ll tear the release sheet in two—but that’s not always the case. If you don’t have one of those cords to rip the release sheet in half, what you can do is use a brand-new knife blade and lightly drag it across the release sheet, just scoring it through the face—and then you can remove just part of that release sheet. You have to make sure you get it nice and tight in the inside corners; otherwise, when the window goes in, if you’ve bridged that corner with the material, it can rupture when you place the window in.