If you go to www.westsystem.com
and look under products, I believe the "A" sized kit should be plenty for you. You'll want slow hardener becuase the cure time will make it easier to do a large area with one batch of epoxy. Wear tight fitting rubber gloves as some people have (or can develop) an allergic reaction to uncured epoxy. The mixing ratio depends on what brand of epoxy you buy so be sure to read the directions and measure carefully. (My friend has built a custom scale balance just for epoxy since the brand he uses is like a 53:104 ratio or something bizarre like that.) You'll probably want to flip the boat upside-down first, then use a grinder to make a nice edge all the way around the damaged area to fill so you know exactly where to fill and where not to fill. Then mask off the areas of the hull you don't want to get epoxy on.
When you're ready for the epoxy, mix the resin and hardener, (a clean plastic cup and a new tongue depressor will work fine for this) then slowly add and mix in flox until it's a consistancy you can spread. Actually more like peanut butter than spackle. YOU ALSO NEED LOTS AND LOTS OF PAPER TOWELS. Lots.
Spread this over the area to build up and use a small rubber squeegee (like for caulking) to do your best to work out air bubbles during application and create an even seam with the surrounding gelcoat. You can also use an adjustable-setting hair dryer (set on high heat but low fan speed) to soften and further shape the epoxy as it begins to cure. Let it cure overnight, then sand the surface to your satisfaction. You can buy epoxy tint to color it but it might be hard to color-match to your hull, or paint over it if you're so inclined.
One thing to beware (or just be aware) of - epoxy warms up as it cures. The stuff you've applied to your boat will have plenty of surface area to cool, but if you make too *much* then the stuff in the cup will begin to heat up pretty good, so once the epoxy is mixed you're working against time, see? Don't mix it and then go get a soda and answer the phone. lol The stuff in the cup will begin to "exortherm," that is when it reaches a certain temperature it becomes a positive feedback loop and it'll melt the plastic cup. Once you see bubbles forming IN the epoxy in the cup you might be able to get a tiny bit more out of there but that batch is pretty much finished and you'll have to make more. (That's why I said to use slow hardener - it'll take 20 minutes or so to get to that point.)
See attached pics where we used this method to repair my buddy's damaged Sunfish:
Image of mixing up some epoxy. As I recall we only used about 2/3 of this cup before it exothermed in the bottom. Heh sorry about the focus.
Here you see what epoxy with flox in it looks like. You can't really see the large paper cup very well, but you can see the consistancy of the mixture on the tongue depressor. You can also see how we ground out the gelcoat a little bit to make fairing easier:
In case you were wondering, the damage there was all-the-way through the hull so we ground out all the glass and glued in that oak insert, then laid glass over the top of it. You can also see there the peel-ply on top of the top layer of epoxy. This is another thing you can use to help with fairing.
If it looks like some glass has ground away then you could put some more in under the flox to add strength.
Hope that was helpful. =)~