Start with the articles in the Head Mistress forum
Reference Library (on the forum home page). There are several there that will get you pointed in the right direction about size and how to install. Then I'll be glad to help you with specifics. I see you're in Midland TX...unless you plan to trailer the boat far enough to sail it at least 3 miles offshore, forget installing any y-valves...you cannot legally flush a toilet directly overboard or dump a tank in any US waters...you have to be 3 miles out to sea...and in many states it's even illegal to have one on non-navigable intrastate waters.
