There are all kinds of tools for extracting the lower chrome drain in the bottom of the tub, but many plumbers use a simple sharp steel chisel. Remove the plug in the bottom of the tub, leaving the chrome ring that seals the shoe to the bottom of the tub. Turn the basket counterclockwise to remove it from the shoe. A sharp chisel will notch the face of the chrome and turn the basket 1/4 turn (but don't hit too hard or you will cut right through the brass). Repeat every quarter turn. Eventually the basket will come out of the tub and you can remove the parts below the tub and replace them with a new waste and overflow. Follow the directions in the new waste and overflow box for installation.