Step One: Using a browser, go to steamgriddb.com

Step Two: Search for your game, you would be surprised at what games you can find on here!

Step Three: You need four different pieces of art: two grids (one wide, one tall), one Hero and one logo. These are all the different art types. When you find one you like, click it and then press download. Then right click and save it to an easy to get place

Step Four: Go to steam, then to your non steam game and click on the gear -> Properties -> Customization

 Step Five: Now you need to set all of your art, put the correct art types in the correct spaces. Dont worry, its easy to judge. For example, your cover is your tall grid, your logo is your, well, logo.

 Step Six: Thats it! You can delete the artwork off your steam deck too if you want and it wont change a thing!