Step 1: Cut foam
Building hot wire:
The wire will be best if you can find 1/32" stainless steel cable to cut with. In lew of this, bicycle shift cables are OK, but leave a pattern which needs to be sanded off. Stainless steel wire also works, but it's hard to get it strung on straight. The cable should be mounted to some frame which is very springy, as the cable lengthens a lot when it heats up. The springness lets you set the tension when the wire is cold, and have the same tension when it is hot.
Using hot wire:
Put a voltage over the hot wire cutter which makes it cut smoothly through the foam, leaving little foam fibers behind it. The wire should smoke just slightly once it's cut through the foam. Don't try to force it. It is cutting with heat, so pushing hard will make the wire bend and not speed up the cut. A bent wire means you get the wrong shape. Never hot wire a piece with one person. Get one person on each side of the wire. Mark points on the shape which you want to make sure line up. Number them, so both people are going the same direction around the part. Speed up or slow down to match the other person.
I use spray adhesive to stick the foam to the templates. The propellant in the spray dissolves the foam, so spray lightly from a long distance (1 ft?). Let the propellant evaporate, and then stick the template on.
Step 2: Build a flat table
To make your table flat, build it with many legs. Then use a tight string to define 'flat', and shim the legs until the tabletop matches the string. I neglected to flatten my table, and now my boat has a downward kink in the rear.
Step 3: Mount and Sand
I made a centerline down my table with a taught string, and mounted all the sections along this. This worked well except that my table curved up slightly at one end, so now my boat has a bit of camber at the tail.
Sanding is equally important. Remember that bit about making a constant curvature hull shape? If you do this, you can make a giant sander to sand with, which will get rid of all of the little ripples and bumps in the hull. I made mine about 4 ft long and just walked up and down the boat with it to shape. This is way easier than sanding and sighting to get the shape.
Step 4: Fiberglass
I made my hull with four layers of glass. One (or was it two?) is unidirectional along the hull, two are bidirectional along the hull, and one is bidirectional at an angle. In hindsight, I would have ditched one layer of bidirectional cloth for another uni layer. The cloth weight was roughly 7 oz, 'rutan fabric', which is meant to be good for wet hand layups. This stuff is still my favorite fiberglass due to it's workability. It can be found at Aircraft Spruce.
Learning to fiberglass takes a little practice. It's basically just a process of getting some liquid in a cloth, and getting surface tension to hold it to something. If you do a lot of layers at once, surface tension may not be enough. If you do tight corners, surface tension won't cut it, so avoid both of these things, especially in combination. If you need to do sharp corners, use a touch of spray tack to stick the fiberglass cloth on, and then wet it out carefully. The spray tack holds the cloth in place, even as the epoxy is curing. I would recommend using a peel ply on the top so that the surface may be more easiy painted.
For the bottom of a hull, it should be possible to stick all layers on at once. Get someone to help you with it, as epoxy has limited pot life. Start from one end of your boat, with all of the cloth cut and ready, and work your way to the other end. By the time you are done with the far end, the near end will have set. Eat a lot before starting, as it could be 6 hours of constant work and concentration.
Wear a mask and gloves as it is very easy to become sensitized to epoxy. Many homebuilt aircrafts are never completed because their builders are not careful enough, and end up sensitized halfway though.
Step 5: Sand
Oh yes. Sanding fiberglass is really nasty. If you don't wear a mask, you will wheeze for the rest of your life.
Step 6: Get distracted and build another boat
Step 7: Build top
The top of the shell is under compressive loads, so it wants to buckle, especially around the cockpit. I added a bunch of layers of cloth to bring the load into the rails on the side of the cockpit. On the rails, I used at least 15 layers of glass to form stiff beams which could bear the compression load. It should be possible to lift the boat from bow and stern and not have it break.
Step 8: Sharpen your spoons
After cutting out the foam, bind in divinycell. I used a bunch of microbubble to make sure there were no voids. Divinycell is good for this because it can be heated and formed to make the shape.
After cutting and sanding, lay up the cockpit. The cockpit is in compression, so it needs a lot of very straight bits of fiberglass to keep it from buckling in waves. Don't try to do too much at once. Using peel ply and doing two layups in a tight space is usually a better idea than trying to get all of your thickness at once.
Step 9: Paint, build riggers, ...
I made riggers out of steel. I made a seat out of fiberglass. Paint everything!
Gel-coat is a pain to use, but makes for a nice finish. I used gel-coat and followed up with spray paint to make it extra-yellow, and so I could see scratches.
Step 10: Row!
If you happened to build your boat in a room which it cannot exit from, you may be in trouble. Try removing windows.