Welcome.
In general, the problem with the buses is getting the attention of the driver. There aren't any specific directions on what to do, but in 14 years of traveling to WDW with someone in a wheelchair, here's what we do:
1) DD in her wheelchair and one person wait for the bus near where the back door of the bus will be when it stops. Wheelchairs are put on the bus at the rear door, so it speeds things up for you to be waiting there.
When we are at the resort, and all the busses will be coming to the same stop, they give kind of an exaggerated head nod or shake to tell the driver whether we want to go to that park or not.
At the parks, the bus stops only go one place, so you don't need to signal the driver about whether you wnat that bus or not. You just have to do things (waves arms, etc) to make sure they see you.
2) If there is a line, the rest of our party waits in the line. Sometimes this is really important becasue the driver may not have seen (or in a few cases, ignored) DD's wheelchair. We let the driver know that we have someone in a wheelchair who needs to board.
3) We try not to leave the parks at busy times. After Illuminations, for example, we take our leaving the park, so the first few buses are loaded and gone before we get to the bus stop. If it is busy, our whole party stays with DD instead of waiting in line. We don't want to have her and DH put on the bus and me and other DD left behind.
4) The drivers need to load wheelchairs before other people get on. This helps with manouvering the wheelchair because you don't have to worry about the other people. It's also helpful because some of the seats have to be flipped up to make room for the wheelchair. If people are sitting in them, they don't always react kindly to being asked to move.
5) If we can see the bus is going to be full and people will need to stand, we usually stand, even if we got on before all the seats were full.
Other people might have different hints, but this is what works for us.