The rear deck rests on the hull edges. See the earlier page o n cutting it to shape.
Use a scrap piece of 1/4" to check the angle of the bulkhead and keeping clear of the main mast. Cut the front edge of the quarterdeck at a 12 degree angle to slope the bulkhead.
Use some heavy paper or cardboard for a template and trim to fit the hull edges. Sand the sides of the bulkhead to fit the curve of the hull - it doesn't have to fit perfectly, but a slight angle trying to match the shape will look much better than a straight cut.
Make the ladder and doorway the same way they are built on the foredeck.
Add some scrap blocks cut on the same 12 degree angle to help support and reinforce the bulkhead.