Install New Window
If a section of the exterior siding had to be removed, be sure to put felt wrap back up to keep moisture out. Then apply window wrap to the exterior framing of the window. Start the window wrap installation at the bottom of the window and work your way up.
Install the window from the exterior by pushing the window up until it is flush to the frame. Screw your window into place; make sure you are able to anchor it to a solid piece of wood. Adjust the window until it is plumb and square by maneuvering the screws and adding shims to areas that need adjustment. Once the window is in the correct position, from the exterior, use screws or nails to secure it into place.