My concrete floor has a drain already there. An old metal thin flang with a piese of pvc coming up for the composite pan that was in there. Im going to pour the floor and tile it how do I prep the drian

I can't do better than this guy so here you go, http://www.hoagy.org/house/HowToBuildShowerPan.html

His drian is already in the floor. Mine is to but it is ready for a com. Pan.

Have you bought your shower pan yet. Fiberglass pans have a drain hole with a rubber collar that fills the gap between your 2" PVC drain pipe and the pan. Initially the PVC sticks up above the fllor of the pan while you set the pan in thinset. Once the thinset has cured you cut the PVC right at the top edge of the black coolar's height. Grease the collar and shove it in place.