I did this before for a friends boat. There was a wiring diagram floating around that was a good reference as to how to wire the relays. Make sure to get the waterproof relays. I made the mistake of using standard relays from Autozone and they corroded and failed after about a year. Autozone sells the waterproof relays also, but most sales associates at autozone will have no clue what you are talking about when you go ask for a relays - they always want to know what kind of car its going in.
I reccomend getting the relays from the parts express website listed above, and also order the harnesses (they will make the wiring job much easier).