Step by step:
1) You will receive tags from Magical Express (DME) to put on your luggage. Make sure that you put them on all the bags you will check.
2) Check your bags on your home airport as you normally do.
3) When you arrive in Orlando and exit the plane, take the airport monorail and go to the main terminal.
4) Once there, head to the 'B' side (look for a huge neon "B"), then take the escalators / elevators to Level 1, and head to the DME desk. You don't need to claim your luggage. DME will handle it.
5) At the DME desk, check-in. They will then direct you to the correct line for the bus to your resort.
6) You will line up and then board the bus. Depending on which resort and time of day, you might have to wait for other passengers.
7) When the bus leaves, the driver will inform you the order in which the resorts will be visited and guests dropped off.
8) Once you arrive at your resort, proceed to check-in there as you normally would.
9) Your bags will arrive at your resort later. You do not need to be in your room to receive the bags. They will put them there if you are not present.
Again, depending on time of day and resort, the average experience everyone is reporting is around 60-70 minutes from deplaning to check-in. Bags are received between 1-3 hours after check-in, maybe less. If you arrive late at night, you can call bell services and let them know that you want your bags delivered the next day, if you so choose.
As for the car seat, I'm not sure. If you are bringing one from the plane you can use it, but I believe it's not a requirement on the bus.