Do everything that Steve said!
Then, consider the following... (I just found it to be an interesting point in "relativity".)
Have you ever been on a Ferris Wheel? Have you ever noticed how the operator runs the controls? He does it by "eyeball".
A little forward, a little backward, whatever it takes to align the gondola to the stationary loading platform.
In that case, the platform is the reference point. That is, position adjustments are made relative to the platform. You make the necessary adjustments to bring the Gondolas TO the Platform.
Now... let's switch our frame-of-reference. Instead of bringing the free-moving gondolas to the stationary platform, envision that the gondolas are stationary and the platform is free to move. In this case, you make the necessary adjustments to bring the Platform TO the Gondolas!
You might be able to see that this is kinda like a continuous un-ending elevator. It doesn't suit our purposes. So, open the Wheel and lay it flat. Now, the operator can move the platform from one gondola to another as he SEES fit.
The whole point of this is to get you to SEE how the operator can SEE all of the gondolas and the platform.
So... Let's turn the whole thing on-end and rename it...
The gondolas represent floors in a building. The platform is the elevator. The operator can SEE the whole thing!
The operator can respond as he sees fit according to hand signal that passengers present.
A wave of the hand gets the operators attention, while showing two fingers indicates the desired destination.
Since YOU are the operator, you can see all of the passengers and their signals. In YOUR mind, it's not such a hard thing to handle. You should try to keep the operation effecient. Once you start in one direction (up or down) maintain that direction until there is not a need to maintain that direction. be sure to let the passengers know which way you are going before they get on.
Simple.
Now, put yourself and the controls in a shed with no view of the elevator. What do you need in order to operate?
This is not so simple, but ain't so tough either!