Remove the garbage disposal cable from the electrical socket, if it uses a power cord. However, sometimes the disposal unit is hardwired to the electrical wiring system of the house. For that you will have to turn off the circuit breaker for the kitchen, however, if you cannot locate the specific circuit breaker, you can always turn off the main circuit breaker.
Place a bucket underneath the unit at the point where the pipe meets the side of the disposal unit to the sink drain line. The purpose of the bucket is to catch any left over water in the pipe.
Use a pipe wrench to loosen the nuts of the drain pipe. Always remember that the nuts always go loose in the counter clock wise direction. You can also use slip joint pliers instead of the wrench to open the drain pipe. If the nut does not come off easily, use an anti rust spray, such as WD-40 to ease the joint. After opening the joint, pull both pipes away from the disposal unit.
Empty the contents of the pipe into the bucket. Remove the pipe at both ends, if it is clogged heavily and take it to the sink and clean it with warm water. You can insert a piece of iron rod to remove waste materials which do not let go of the pipe that easily.
You can also use an air blower to clean the pipe if it is not easily removable. Pour WD-40 in the sink, and run the tap water for a while so that it reaches each nook and corner of the pipe. Now, place the drain pipe back in position and lock in its place after threading the joint.