On your device, make sure you are connected to the WiThrottle server network.

Start the Engine Driver app and wait for your server's address to show up in "Discovered Servers".

Click on the discovered address, or you can enter the address and port of your WiThrottle server directly. (Using the values shown on the WiThrottle window.)
NOTE: see [here] for wifi connection issues.
There is also a "demo" server at jmri.mstevetodd.com, port 44444 that you can connect to for testing.

On the throttle screen, click on the "Select" loco button for each loco.
On the next screen, click on a loco from the Roster or Recent lists, or enter the loco address (verify short or long), and press "Acquire". You will be returned to the Throttle screen.
Repeat using another "Select" button to acquire another loco.

Operate your loco using the slider, or volume control, or optional buttons for speed, and the reverse, stop and forward buttons to control direction.
To release all locos, just press the Back button and exit the app. This will also stop your locos (can be overridden in preferences). You can reassign the volume control by clicking on the speed indicator (the small "v" shows which has control).

To create a consist "on-the-fly", simply select additional locos for the same throttle, and select which way the new loco is facing. Direction and speed will be sent for all. Release will release all for that throttle.

Press your phone's Menu button to access screens for Turnout, Route and Power control, as well as adjust settings and set numerous preferences.
You can also "fling" left or right to jump quickly back and forth between the Throttle, Routes, Turnouts and Web pages (use Preferences to choose)