Prepare the rough opening

• 1). Measure to make sure the rough opening is 1/2 inch taller and wider than the window. This will leave a margin of 1/4 inch all around the window when set in the center of the opening.
  
  
• 2). Cut the water resistant barrier, such as Tyvek House Wrap, along the header, then the bottom along the sill, then make one vertical cut in the center. Fold the left and right flaps into the window opening and staple to the sides of the window opening. Trim off the excess. Staple the bottom to the outer edge of the sill. Cut 8-inch vertical slits at the corners of the header, fold the resulting top piece up and tack it in place.
  
  
• 3). Cut a length of sill flashing tape 12 inches longer that the width of the sill. Position it flat on the bottom of the sill and six inches up on each side. Extend the tape 1 inch beyond the exterior face of the wall. Using the knife or scissors, cut the corners so that you can fold the outer edges against the exterior of the framework. Staple in place.
  
  
• 4). Cut another length of sill flashing tape 12 inches longer than the width of the sill. Position in the sill as in step 3, but toward the interior of the wall. Overlap the first tape by at least 1 inch. The tape doesn't have to extend all the way to the interior edge of the wall, and should not extend beyond the interior edge of the wall. Staple in place.
  
  

Setting the window

• 1). Take two shim shingles and position one flat at each end of the sill with the fat ends flush to the exterior wall. This will leave the thin ends sticking into the room. Leave 1/4 inch from the sides of the framework to each shim. Use the level to make sure the shims are level across the sill. If not, adjust the shims, or add another shim, until they are level. Once level, tack the shims in place with small nails. Trim any parts of the shims that stick out over the edges of the sill.
  
  
• 2). From the outside, insert the window in the rough opening so the bottom edge of the window frame rests on the shim shingles and the nailing fins overlap the water barrier on both sides and bottom of the sill, but not the top.
  
  
• 3). Have a helper on the inside of the room center the window in the opening. Then, from the outside, nail in 1 roofing nail on each end of the top fin to hold the window in place.
  
  
• 4). On the bottom fin, nail in a nail at each end, but don't nail them in all the way. Check to make sure the window opens and closes easily. If not, remove the bottom nails and shift the window to one side or the other until it opens easily. Once it works well, nail in the bottom fin and nail into every fin hole all around the window.
  
  
• 5). Cut two pieces of flashing tape 4 inches longer than the sides of the window frame. Overlap the fin by 1/2 inch and extend the tape vertically 2 inches above the top and bottom of the frame. Press firmly so that it adheres well.
  
  
• 6). Cut a piece of flashing tape for the header, long enough to overlap the side flashing tape by 1 inch on each side, and the top fin by 1/2 inch. Position the tape, and press firmly so that it adheres well.
  
  
• 7). Fold down the 8 inch piece of water barrier that you tacked up above the header in step 2 of section 1 so that it overlaps the flashing tape on the header. Cut small pieces of flashing tape to cover the vertical cuts in the water barrier and affix them.
  
  
• 8). Apply the insulating foam sealant to fill the gaps between the interior window jamb and the framing. Allow to dry for 24 hours.
  
  
• 9). This completes the window installation process. Now all you have to do is finish the interior and exterior walls, and trim out the window inside and out.