Step 1 : Prepare the rough opening

Follow the window manufacturer's direction for the correct size and measurement of the rough opening and cut it to those exact dimensions. Make sure that the opening is level, plumb and square. Check for squareness and dimension by measuring diagonally across, upper left to lower right and upper right to lower left; if the measurements are the same, the opening is square. Make sure the sill plate is level by using a carpenter's square and level to confirm it is plumb and level. Test fit the unit to make sure it fits.

Step 2 : Apply extension jambs or drywall return

Position the tongue on the drywall return or extension jamb into the groove on the frame. Hold firmly to avoid tipping and fasten in place using 1 1/2-in.(4d) finish nails.

Step 3 : Install the unit in the opening

Lift the window unit into the rough opening from the exterior. From the inside use shims at corners of the sill under the side jambs to make it level and center the unit.

From the exterior use 1 3/4-in. roofing nails to secure the unit in an upper corner.

Step 4 : Check for squareness and fasten

Recheck for squareness and dimension by measuring diagonally across, upper left to lower right and upper right to lower left; if the measurements are the same, the opening is square. If the dimensions are not the same, adjust with shims as required and fasten the remaining corners.

Measure across the head, center and sill of the window unit. Center dimensions should match the head and sill dimensions. Shim or block the side jambs, if necessary. Insert shims at midpoint behind the side jamb between the unit frame and rough opening. Fasten shims about every 18-inches. Secure the unit to the wood frame rough opening using 1 3/4-in. roofing nails spaced every other hole.

Step 5 : Insulate and seal the unit

Fill any void cavities with insulation by stuffing it between the framing, extension jambs and rough opening on all the sides. Don't overfill it. Then apply a silicone sealant around the exterior perimeter of the window after the siding or other finishing trim is installed.