Cut the seal between the window stops and the window frame. The window stops are wood or plastic strips that hold the window in place. Window stops are roughly 1 to 1 1/2 inches thick and are nailed to the window frame. Slide the pry bar or putty knife under each window stop and pry away from the window frame.
Remove the parting stop located from the groove in the window frame with the putty knife. Cut the balance cords if they are accessible. Remove the lower window sash from the window frame. Lower the upper sash and pull it towards you to release it from the window frame.
Lay a two-inch wide bead of caulking on the base of the window frame. Start the bead at the left vertical side of the window frame at the exterior window stop. Drag the caulking gun across the base of the window frame. Run the bead of caulk towards you two inches and continue the bead across the window base, remaining two inches away from the exterior window stop. When you reach the other vertical side of the window frame, run the bead of caulk back to where you started.
Set the pre-hung window insert into the window frame. Make sure that the pre-hung window insert is tight against the exterior window stop. Run screws into the pre-drilled holes in the window insert. Do not tighten the screws all the way. Insert the shims between the pre-hung window insert and the window frame. Use the level to level the pre-hung insert. Tighten the screws to secure the window.
Place the batt insulation around the window insert. Do not over pack the insulation. Over-packed insulation could bow the upper frame of the window insert.
Install the interior window stops. Use the hammer and finishing nails to secure the window stops to the existing window frame. Test the operation of the window. If the window binds, check the window for square. To check the window for square, measure diagonally between two opposing corners. Then measure between the other two corners. If the measurements are equal, the window is square.
Caulk around the window after it is square and operates smoothly. You will need to caulk the area where the window stops meet the pre-hung window insert. Move to the exterior of the window and caulk the exterior window stops to the pre-hung window inserts.