The hull lines show a cross section of the boat.
Sometime the left side shows the bow to the mid point of the boat, and
the right side shows the stern to the mid point of the boat.
If you make a copy of the Hull lines, and fold the image in half, and cut, say, frame number eight, then unfold the paper. The cut out will show you a complete section of the hull at that point, frame eight.
If you look at a side view of the boat, or hull lines, there should be a set of numbers underneath the image that correspond to where each frame number sits in relation to the length of the ship or boat.
http://www.pt-boat.com/originalhull/pt593.html
If you make a paper cut out of each frame, you can then spray mount the paper cut outs to plywood, and cut your frames out of the plywood using a coping saw, band saw, or jig saw. You would then cut out a keel from the plywood using the side view, mark that for the fram points, and stand each plywood frame onto the keel. Once you get this done, you are well on your way to scratch building your PT boat.
[8D]