You can simply bind inc/dec actions to keys of your choice.
Auto-detect feature will automatically detect if you are dead using death_screen.png.
It may use ~4% of your CPU, but you can lower it by setting higher interval in stuff/settings.json (not recommended).
If you want to manually set the death screen for other games, you can do it by changing death_screen.png in the stuff/ folder.
Keep in mind that the image should be in your native resolution e.g. 1920x1080.
You can set a custom sound effect that will be played when you die.
To do so, place your sound file in the stuff/ folder and set the death_sound_effect field in settings.json to the name of your sound file (e.g. "stuff/death_sound.wav").
You can also set it to null to disable the sound effect.
If you have any issues with the death counter, try launching it with administrator privileges.
If it still doesn't work, you can report the issue and describe your problem in detail.