how do i install a garbage disposal properly?

Power off at the breaker first, and check whether yours plugs in or needs hardwiring - that decides whether you need an electrician. Then plumber's putty under the new sink flange, push it down, mounting ring on underneath and tighten it up. Hang the unit on the ring and twist till it locks. If you've got a dishwasher, knock out the plug inside the dishwasher inlet before you connect the hose. That one gets forgotten constantly and the dishwasher just won't drain afterwards. Connect the discharge to the trap, run the water and check every joint with a dry tissue.