Pry off the trim located on both the interior and exterior of the old window with a pry bar. Cut along the sides of the frame between the window and the jamb with a reciprocating saw to break the window free from the screws.
Cut the sill along the bottom frame of the window directly in half and push the window out of the opening to remove it.
Clean off the area where the old window was to remove any dirt and debris and measure the opening to determine the exact size window you need to install. Measure both the length and width of the opening three times and record the smallest measurement.
Set a level along the sides and base to make sure everything is plumb and square.
Apply flashing tape along the sill. Roll it out over the bottom of the sill and a few inches up each side and nail the corners along the outside of the opening in place.
Run a 1/4-inch bead of silicone sealant along the exterior of both sides of the opening and along the top. Place the silicone sealant in a caulk gun and push down on the caulk gun until the sealant begins to come out.
Center the window in the opening and push it into place from the exterior. Press down on the sides and top of the nailing flange so it can adhere to the silicone sealant.
Insert a nail in each corner of the nailing flange to hold the window in place. Add shims to the sides of the window to level it out.
Set a level down again to make sure it is plumb and square. Insert nails with a hammer into all the openings on the nailing flange to secure it to the wall.
Apply flashing around the sides of the window with silicone sealant. Cut the excess shims off with a utility knife.
Add sealant to all sides to ensure everything is secure.