Draw out the window opening on the wood. Using a small drill bit (1/8" approx.) drill a hole in all four corners being careful to not drill outside the lines. Disassemble your coping saw and thread the blade through one of the holes. Reassemble the saw and start cutting along one of the lines. Cut out the wood inside the window opening. I used a file to smooth the cut edges and to square up the round hole in each corner. Don't worry about being perfect. The flashing around the window will cover a multitude of errors.
John