Tighten the connections between the different drainpipes in the tub drain kit. Use your hands to make the connections as tight as possible.
Place the assembled drainpipe system under the tub to see how it fits with the waste pipe. If the bottom pipe on the drain pipe assembly overlaps the drainpipe, use a marker to mark how much of the pipe needs to be cut away.
Trim the end of the drainpipe assembly at the marker line, using a hacksaw. Place the assembly back under the tub, and tighten the connection between the bottom of the drainpipe assembly and the waste pipe.
Position the overflow drain cover over the overflow drain opening, threading the stopper's linkage down the overflow tube. Drive the screws through the holes in the drain cover and into the screw holes in the overflow tube.
Spread silicone caulk over the bottom of the drain's lip, and then thread the drain into the pipe opening through the hole in the bottom of the tub. Insert a drain key into the drain assembly's opening, and turn its dial to expand the drain key so it sits snugly in the drain opening. Clamp an adjustable wrench to the top of the drain key, and twist it clockwise until you feel resistance.