For the exact commands you have to put to make the specific games work, please refer to the README.txt that comes with DemulShooter.
You extract DemulShooter somewhere ( e.g. program files), open the app like you most likely did, configure your settings and close the app.
Then you open a cmd window, change the directory to where the DemulShooter.exe is located, and run Demulshooter with typing the command:
DemulShooter.exe -target=ttx -rom=mgungun2
You`ll have to swap out the ttx and mgungun2 for the ones needed for your game -> take a look at the ini.
If you pressed enter, "nothing" should happen, but you`ll see DemulShooter.exe running in the taskmanager.
Then just go ahead and launch the game you just stated in the demulshooter command. Make sure there are no spaces in the commands, else it won`t work.
have fun