I would absolutely use on-line check in, if available. It's even faster than using the Kiosks. (Not to mention that on my last trip (April 30th) all the Kiosks were down so I had to wait 45 minutes in line.
Basically, you enter all your info -- name, address, drivers license #, credit card info -- accept/decline extra options and then print your paperwork. Once you get into MCO, you take that paperwork directly to the garage, pick out your car and stop at the booth on the way out where they will ask for your paperwork, credit card and driver's license.
Go for it!! (I can't remember how early you can do it, but I'm sure if you started the process, it would tell you if it was too soon)