I finish with a vertical cut from the top down the middle.
Note: Don't make an "X-cut" at the top; this will direct water into the structure.
After removing the release paper, I make sure the membrane is centered in the opening...
Note: Flashing membranes are meant to be installed at temperatures of 45 degrees or above. In colder weather, I keep the flashing in a warm place and cut pieces as I need them.
The flashing has a memory, so to prevent it from curling back, I drive a cap nail at the outer edge to hold it in place until the adhesive cures (24 to 48 hours).
Flash the Jambs and Head
Starting 2 to 3 inches above the window head, I apply flashing membrane over the jamb flanges, letting the tape extend at least to the bottom of the sill flashing.
... lapping it over the top of the jamb flashings.
Note: Applying the head flashing directly to the sheathing provides another level of protection. If any moisture were to find its way behind the housewrap and seep down to the window, it would encounter the head flashing and be directed over the windowhead flange, not behind it.