Disconnect the bathtub overflow drain cover by removing the screw(s) holding it in place. Unscrew the tub drain from the bottom of the tub with a tub shoe removal tool. Cover the drain with an old rag to prevent debris from clogging the drain. Brake the wall around the tub at least six inches from the tub itself. Remove the old bathtub and recycle it, if possible.
Mark the location of the new center drain according to the manufactures instructions; not all tubs have the same exact drain location. If you have a concrete floor, use a concrete cutoff saw to remove a four- to six-inch path from the old end drain to the new marked center-outlet drain location. If you have a conventional floor with a crawlspace underneath, you only need to cut a 12-inch hole in the floor with a circular saw. This will allow enough room for the new center drain to connect to the bottom of the tub.
Cut the existing p-trap off the tub drain line under the floor with a reciprocating saw. Install a rubber coupling if your drain is made out of metal. If your drain is made of PVC, glue a coupling on the open end of the drain line. Install a new p-trap and drain line under the marked location of your new center-outlet tub drain. Install a trap adaptor above the p-trap within six inches of the floor; this is for the new waist and overflow to connect to the tub drain.
Install your new center outlet bathtub according to the manufacturer’s instructions. Install the new center outlet waist and overflow. Insert the drain tee into the trap adaptor just below the floor. Slide the tub shoe into the side of the tee and align the drain inside the tub. Insert the over section into the top of the tee and connect the overflow plate from inside the tub. Install the drain in the tub shoe from inside the tub with silicone caulk or plumber’s putty. Tighten the drain with tub shoe tool. Tighten the connecting nuts of the tee by hand from behind the tub.
Close the tub drain and fill the tub with water. Allow the water to set for at least an hour to test the drain seal. If there are no leaks at the bottom drain, continue filling the tub until the water runs over and down the overflow drain. Release the tub drain and check the all the visible pipe connections under the tub while the water is draining. If there are no leaks, you have done a great job. If there are leaks, don't panic -- just remove that connection and reseal it.