Add Shims and Insulating Foam
1. Shim four corners and check window for plumb.
2. Caulk gap on sides and top.
3. Insert screws in pre-drilled holes in window frame.
4. Place corner bead of sealant around perimeter of window frame where new frame contacts original window frame.
5. Apply insulating foam sealant between window and rough opening.
6. Re-install inside stops.
7. Caulk inside perimeter.
8. On exterior, fill gap under new frame with insert or install flat vinyl trim, if necessary.
Place, DO NOT HAMMER, shims.