Hi!
It's pretty easy, actually.
Make sure the DME knows in advance that your Aunt will be bringing a personal mobility device, and that she will need to use the lift.
When you arrive at MCO, check in at DME on the lower level. You will be directed to a special HA (Handicapped Accessible) line.
When it's time to load, you will be told which bus to go to. You will load first (just like at the parks). The driver will bring down the lift. It is located on the passenger side at the rear of the bus. It is actually concealed quite nicely when not in use, so many people don't even know the lift is there.
Once the lift is on the ground, your Aunt will have to BACK the
ECV on to the lift. This is probably the hardest part of the entire experience for most people. Depending on the lift, and the pavement, there may be a small "bump" as she backs on to the lift. As long as she lines up nicely prior to backing on to the actual lift platform, there will be no problem.

Once the ECV (and your Aunt) are safely in position on the lift, the driver should put a safety strap across the opening of the lift, and the lift will be engaged.
The lift process itself is smooth, if a bit slow LOL.
Once the lift is at the top (level with the interior floor of the bus) then the driver will go inside to help your Aunt BACK the ECV into the bus. It will be a straight shot back into the bus at that point - as long as she doesn't turn the tiller she should be OK. She will back the ECV off the lift, and then it will be parked right across from the buses bathroom door. (the rear row of seats just in front of the bathroom door on the passenger side will have been removed, exposing the standard Disney-style tie downs that we all know and ?love?)
The process is reversed at the Resort - the ECV will be loaded out onto the elevated platform, the platform lowered, and the rider can then begin their WDW adventure!
Some things that might make this easier for your Aunt are to practice, practice, practice her "precision driving" as much as possible prior to getting in the line at DME. By this, I mean that if she can control the forward and/or rear motion of the
scooter in a very slow and precise fashion, it will make the entire process much easier and less stressful for her. If she can drive one of the big lumbering beasties that Wal-Mart has for ECV's in a relatively good fashion, she will likely be fine. It doesn't take most people very long to figure out how "touchy" the throttle is on an ECV; my personal mobility device is pretty smooth, but some I have rented have been kind of finicky.
Just as an FYI - I personally tip my driver $5 both coming and going for helping me get on the bus. It's not required to tip them - I know it's part of their job - but many of the DME drivers are so awesome about helping, and being patient during the whole process that I don't mind giving them a little tip.
Hope this helps - we will be running down next week for a quicky little trip, so if at all possible, I will have a family member snap some pictures so you can see how it looks.