Look for plans that have hull forms very close to what you want to build, then modify them the rest of the way. If your design is totally unique, build a small half-hull in clay, then bread-slice it into sections and trtace them onto paper. You can scan and enlarge the pattern to your preferred scale. Before you slice it, trace the side view off, too, marking where each slice is made so you can cross-reference your section views. Use the half-side sections to trace off a mirror-image exact duplicate of the other side and the two joined in the middle will be a full hull section.
To build the hull, make cardbord or flat wood cutouts of the sections, then attach them along the length of a piece made from the side tracing. Add stringers along the corners and angles to stabilize it all and then plate over it with styrene plastic or balsa wood. Put a glossy finish on it with dope or primer and use the finished product as a plug to make a mold from which your finished hull will come. This way, you can also save the mold and make as many hulls as you want.
Another way, used very successfully by our own Jeff Herne, is to buy a card model and transfer the shapes to styrene or wood and use it as a plan set.