Place a packer in line with the top pilot hole, in between the frame and the wall. Then check that the side of the window is sitting plumb and flush with the internal wall. When you are sure everything is in place, drive screws through your pilot holes and into the frame of the house. To give your job the best finish, countersink your screws so they sit below the surface of the timber. That way you can cover them with putty before you paint.