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.
Slope the SillI fasten a piece of beveled siding to the rough sill, with the thick edge toward the interior, to direct moisture outside.
Flash the SillAfter taping the housewrap to the studs, I install the sill flashing membrane, cutting it a foot longer than the width of the opening so it can extend up the jambs 6 inches. Note: I prefer to use Tyvek FlexWrap, a butyl-based flashing membane. It has the self-healing characteristics of generic peel-and-stick but can also be stretched to form seamless corners.
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).
Caulk the OpeningBefore installing the window, I apply a heavy bead of elastomeric latex caulk at the jambs and the head, but I never caulk the sill flange area. Note: Should any moisture find its way into the rough opening, this caulk-free sill flange, coupled with the sloped sill, will provide a weep area for water to escape.
Install the WindowNext, I install the window, driving roofing nails through the preformed holes in the flanges, spacing them about 6 inches apart or per the manufacturer's directions.
Flash the Jambs and HeadStarting 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.
Skip-Tape the HeadI "skip-tape" (apply short pieces of tape with gaps in between) the lower edge of the flap in place. Note: These skip-taped gaps act as weeps, giving any moisture that gets to the window head a way to escape.