go to

https://ja2-stracciatella.github.io/

download the windows installer, then add it to steam as a non-steam game, run the application (remember to set up the compatibility tool to Proton Experimental), then find your JA2 directory, i recommending installing JA2 on an SD card, makes it easier to find, the SD card will show up as drive E:

then after you finish installing, make sure ALL of JA2-Stracciatella were installed on the same directory as your game directory, you might need to move the contents of the installation, just make sure you overwrite the ja2.exe file

go to the JA2 directory and add ja2-launcher.exe as a non-steam game (remember to set the compatibility tool) run the executable and setup your JA2 directory there, also, setup the resolution to 960x600

while JA2-Stracciatella supports lots of widescreen resolutions, the UI does not scale, this is a problem specially when it comes to managing squads and using the laptop, 960x600 is widescreen and the UI will fill up most of the screen, i also recommend enhabling FSR after doing this, just press the "..." button on your steam deck for that

if for some reason the launcher isnt saving your video configuration, you can manually edit the config file, its located here:

/home/deck/.local/share/Steam/steamapps/compatdata/12370/pfx/drive_c/users/steamuser/Documents/JA2/ja2.json

on that file you can setup resoltions like this:

"res": "960x600"

and your game directory like this

"game_dir": "E:\steamapps\common\Jagged Alliance 2 Gold"

this is a json file remember to add a "," at the end of each line except the last one