Remove the trim from around the double hung window with a putty knife, chisel or small crowbar.
Lift the double-hung window sashes out of the window frame.
Place the single-hung window into the window frame. If you had the new window custom manufactured to replace the old window, it should perfectly fit into the space. If you are fitting a window that wasn't made especially for that space, adjust the trim or to fill in around the frame, as needed.
Secure the new, single-hung window in the window frame by driving screws through the designated areas in the frame of the window and into the side trim of the window frame.
Replace the strip trim around the new window.