There are two 'sticky' threads on the Transportation board here for Magical Express, but...
The day before you depart, there will be a packet on/under your door with your departure information. It will tell you what time your bus leaves your resort, and that you need to be waiting for the bus with your entire party and any carry-on luggage at least fifteen minutes before this time.
There will also be information on how to prepay for any checked luggage that day (i.e. don't leave it to the last day) if you'll be using the Resort Airline Check-in feature because you're flying home on a participating airlne.
The next day, some time between five and one, no fewer than three but no more than twelve hours prior to your flight, you will go to the RAC counter and check in for your flight. They'll take, weigh, and tag your luggage, and print your ooarding passes. Your luggage willl get sent to the airport and onto the plane so the next time you see it will be at home, and if your time allows, you can go do something until your bus time approaches.
Note that if you miss your assigned bus (a) Disney isn't obligated to get you to the airport; (b) since buses are scheduled based on need, if nobody at your resort has a flight leaving withn two hours of yours, no bus will come along; and (c) just because a bus comes a half hour later does NOT mean it will have room for you.