A seat is a pretty basic need in a kayak. A good seat is the key to comfortably sitting in a kayak for hours at a time. The seat is the primary contact between the paddler and the boat and as such it is the primary tool for controling the kayak.
There are some pretty nice pre-made seats available if you don't feel up to making your own, but I don't think any are as comfortable as this one I make using 3" thick minicel foam. I start by cutting out a 14" x 16" (36 x 40 cm) rectangle. The foam is really easy to cut. I use a Japanese "pull" saw, but have used a bread knife.
The large dimension of the seat is the fore-aft length. I like a lot of support under my thighs to help keep my legs from falling asleep. I start by marking the centerline between between the thighs, I then mark the deepest point which will be about 3 inches from the back of the seat.
Foam is light and the tools I use are powerful. It is easy to send the chunk shooting across the yard, so you need a means of holding the chunk in place. I've found the best think is two-sided tape. I normally use double-stick carpet tape, but I was out so I used some mounting tape I had in my shop.
I then stick to the foam block to a piece of plywood at a comfortable working height. I use a right angle grinder with a carbide carving disk (mine are "Galahad" rotary sanders from King Arthur Tools) but you can do the whole job with a 24 or 36 grit sanding disk from a home center. You don't even need the power tools, the coarse grit makes pretty quick work even wielded by hand. I place the seat where I don't need to bend over much and can securely brace myself. I tend to do this work outside because dust goes everywhere.
I start by sanding a "U" shape in the foam, concentrating on the bottom of the "U" where the butt will sit. This is the deepest part and the bottom of the foam will be about 1/2" (1 cm) thick a this point when complete. With the right angle grinder it is very important to keep the disk spinning against the direction you are moving it on the foam. The friction of grinding creates a powerful pull, and if you allow the grinder to go in the direction it is pullng, it is very happy to take off. The result is a deep divot in the foam. You will probably make a few gouges before you get a feel for it.
I make a few horizontal cuts at the bottom of the "U" and then blend down into this trough with downward cuts along the leg area. The leg cut should be fairly shallow at the top, getting deeper as it approaches the bottom area. The back of the seat should be rounded. Think about the shape of a cast iron tractor seat. The idea is to get the shape to fit. If a farmer could sit all day in hard cast iron, you should be able to make something comfortable with foam. If the shape is good, it won't matter if the seat is foam or cast iron. The beauty of foam is it is easier to shape than iron.
When I have the basic shape roughed out I get my piece of plywood up on saw horses so I can move around the seat better. Again, since you want to be mindful of the direction the grinder is spinning, it pays to be able to move around the seat to a spot where you can comfortably make the motion you need, wit the tool going the direction you need.
After a little more rough shaping to get close to the desired shape I remove the disk from the grinder and work by hand to refine the shape more. You don't want a big, high, sharp ridge running down the middle. It is better to have a bulge to cradle your upper then that flattens out as it approaches your butt.
The proof is in the sitting. Before you get too far along you will want to actually sit down and see how the seat feels. Feel for pressure points and high spots. The seat should support your legs.
I switch to a finer disk as I get closer to done to give the surface a finer suede-like finish. It is possible to use a blow torch to create a tougher surface, but it takes a careful touch and the results are not the prettiest.
The bottom shape of the boat will determine the bottom shape of the seat. With a fairly flat bottom you won't need to do anything to your seat, but this boat has a shallow "V" so I tapered the sides of the bottom a bit. To clean up the edges I used a bandsaw.