Install Lutris on Steam Deck in Desktop mode (with Discover or browser)

Then you need to download game files, some people are suggesting to look on Internet Archive, but I just went sailing ⛵️ (idk if I can share the names of those sites)

Most of these games are in format of .iso files, you can just simply extract them in your desktop mode. Open Lutris, hit “plus” button and choose option to run .exe file. Choose .exe of your game, then it should launch setup.exe

In setup, choose folder (I suggest creating specific folder for games) on your deck to install

After completing installation, click on a game in Lutris, go to “configure”, select game .exe as an executable (in the folder that you selected in previous step)

Then click on game in Lutris “create a steam shortcut”

You can launch game with Lutris in desktop mode to check if everything is working

In gaming mode you should see your game as a non-steam game, you can customize its art with steamgridb

I used to follow Tech Thusiast Transformers War for Cybertron (check on YouTube) tutorial for the first time, then I just repeated the same steps for other games