Once you have downloaded the Decoder Pro software and installed it, simply open the program in the usual manner for your operating system. You should see this or a similar opening screen displayed:
It may take a while for this screen to come up, especially on older and slower computers. Remember that you are essentially running the program on a Java emulator over your native operating system software. New computers can do this fairly quickly, but older ones will feel like they're taking forever. Be patient - it will come up eventually! Even on older computers, once the program is up the response time is quite good.
If this is the first time you have run the program after you installed the software, the Preferences screen will come up automatically to allow you to set up the system for your particular configuration. Also if you change your system configuration, OR if you have your laptop away from the railroad, but you want to play with JMRI there is a simulator mode you may want to try. So click next, and we'll have a look at preferences.