Start with the space under the sink. A disposal that fits the sink but not the cabinet causes trouble on install day, so settle the fit before the unit comes out of the box.
Check the space before you lift anything
Measure the cabinet opening, the clearance behind the drain pipe, and the height from the floor to the sink flange. Compare those measurements to the dimensions in the manual for your unit. Note the mounting style and the wiring setup the manual describes.
Look at the drain and the power source too. Find where the drain trap sits, whether a dishwasher hose will connect to the disposal, and where the outlet or switch is. Write down what you find. The install goes faster when the parts are already known.
Shut off power and mount the assembly
Turn the circuit off at the panel. Confirm the power is off before you touch any wiring, and test the outlet or switch to be sure it is dead. Remove the old disposal if one is there, and clean the sink flange surface.
Set the mounting gasket and flange the way the manual shows. Tighten the mounting ring evenly so the seal sits flat. Lift the disposal into place, engage the mounting ring, and tighten it by hand, then with the tool the manual specifies. Do not overtighten. The gasket should seat, not deform.
Connect the drain and the power
Attach the discharge outlet to the drain trap using the gasket or washer kit in the box. Check that the trap sits level and drains downhill. If the dishwasher has a drain hose, remove the plug from the knockout port only when the hose is ready to connect.
Wire the unit according to the manual, keeping the cord or hardwired connection in the orientation the manual shows. If you are not sure how to make the electrical connection, ask a licensed electrician to do it. Keep the circuit off for the whole process.
Run the test before you close the cabinet
Run cold water at a steady flow into the sink. Turn the disposal on with the water running and listen for a steady sound without grinding or rattling. Check under the sink for drips at the flange, the discharge, and the dishwasher connection. Run it again with a small amount of food scraps. Then turn it off, let it spin down, and check that the drain flows freely.
Record the install date and the model number in your home file. That date is where the service life clock starts. For typical service life and planning dates, see Garbage Disposal: typical service life and when to plan.