To build the deck we join the ends of the gunwales and spread them apart in the middle to establish the shape. Then we put deckbeams in between the gunwales so they hold the shape that we created. What we end up with will look sort of like a ladder that comes to a point at each end.
Step 1: Determine the beam of your kayak
Standard Greenland procedure was to make a kayak 6 inches wider than your hips. My hips are 15 inches wide so my kayak would be 21 inches wide. 21 inches is a good width.
If your hips are narrower, your kayak could be narrower, but unless you are an experienced kayaker, I would not recommend a kayak narrower than 21 inches. Even a 21 inch kayak may feel very unstable to an inexperienced kayaker. If you have never kayaked before, make your kayak 23 inches wide and you will feel a lot more stable.
Of course if your hips are wider than 15 inches make your kayak 6 inches wider than your hips. If you make your boat too narrow, your butt will get pinched. Keep in mind that the sides of the kayak taper in toward the bottom, so that the floor of the boat is narrower than the beam.