Make sure you have all parts listed on the instruction sheet. Turn off the power to the disposal at the circuit-breaker box. It's a good idea to check the amperage of the circuit to be sure the disposal won't overload it. The amperage should be shown on the breaker switch.
Remove the disposal unit by twisting it off of the mounting nut. Dump out any remaining water and debris. Turn the unit upside-down, and remove the plate covering the electrical connections. Remove the grounding screw and wire nuts, and pull the cable out of the unit. You may also have to loosen a strain relief sleeve.
Remove the existing mounting assembly and drain. Unscrew the mounting ring, and push the drain flange up through the drain hole. Scrape off any old plumber's putty from the sink.
Note: If your new disposal is the same model as the old unit, you won't need to replace the mounting hardware.
Place a rubber seal on the underside of the drain flange. If your new disposal doesn't include a rubber seal, you'll need to use plumber's putty. Push the drain flange into place in the drain hole.
Place another rubber seal on the drain flange on the underside of the sink. Attach the metal backup ring, flat side up. Most disposal units use metal mounting hardware, but some units use plastic. Check your owner's manual for specific installation procedures.
If your disposal will be connected to the dishwasher drain, use a hammer and screwdriver to remove the knock out plug. Turn the disposal upside-down, and shake it to remove anything that might be inside. Remove the electrical plate, and gently pull the electrical wires away from the unit.
Screw the strain relief sleeve into place, and push the electrical cable through the sleeve into the disposal. Firmly tighten the sleeve.
Connect the electrical wires, splicing white to white and black to black, then secure the connections with wire nuts. Connect the ground wire to the green screw on the disposal, and replace the cover plate.
Hang the disposal by twisting it into place on the mounting nut. Rotate the disposal until it is properly aligned, then attach the drain arm and dishwasher drainpipe. Run water through the disposal for a few minutes to check for leaks. Then turn the power back on.
Note: The disposal may be heavy, so you might want to build a support base to hold it up.