Start the ADTPro server from your host computer's command line or by clicking on the startup program for your host operating system:
You can modify the startup program to suit your environment. Especially for the Unix variants - it may need a little tweaking to get the correct native serial library connected.
When starting, you will see the following information echoed back on your console as the GUI is coming up:
ADTPro Server version v.r.m RXTX: Stable Library ========================================= Native lib Version = RXTX-2.1-7 Java lib Version = RXTX-2.1-7
Once started, the server program's GUI comes up:

From here, you can select the communications device you want to use at the host end by clicking one of the shiny buttons (besides Disconnect, of course). Note that serial communications has an additional set of configuration options, accessible from the File->Serial Configuration menu item. The first time you use serial communications, the Serial Configuration dialog box will come up automatically to allow you to pick a communications port and speed:

(If you don't have the ADTPro client software on your Apple II yet, take a look at the serial or audio bootstrapping section to get that started.)
When you boot your Apple ][ from the ADTPro disk, it will start a simple menu where you can pick which version of ADTPro you wish to start:

You can navigate among the choices by pressing the first letter of the program, or by using arrow keys and hitting enter to select one.
Be sure to choose a client program at the Apple II end to match the communications device being used at the host end. You can choose which one will start by default by replacing the supplied STARTUP program on the disk. Your very simple autostart STARTUP program could look like this:
10 PRINT CHR$(4)"- ADTPRO"
Just change ADTPRO specified in line 10 to the program of your choice:
For example, to always run ADTPROAUD, the program should look like this:
10 PRINT CHR$(4)"- ADTPROAUD"
When you have the line looking like you want it (remember Applesoft commands NEW to start over, and LIST to see the program) save it back out to the disk: SAVE STARTUP. That program will be the one that starts automatically after booting from then on. If you make a mistake and the Apple complains, "PATH NOT FOUND" when you boot, check your STARTUP program by LISTing it and checking the disk's contents with the CAT command.
However you start it, the main screen comes up:

The best way to test communications is to hit the "D" key from the client to ask for a directory listing at the host. If the client "hangs" waiting for a response from the host, or if garbled data comes back, it will be necessary to check the physical connections, port names and speeds on both host and client ends. The ethernet version of ADTPro (ADTPROETH) often needs a couple of directory queries to find the server after setting up its configuration. If you're using the audio version of ADTPro (ADTPROAUD), ensure the volume setting for your sound card is set to halfway up the scale (i.e. much louder than you'd generally listen to computer speakers).
IIgs Note: users running GSOS are advised to hit the '8' key while booting. That has the effect of starting the IIgs in 8-bit ProDOS (BASIC) mode. The less contention for the serial hardware, the better.