Maybe it's just me but I didn't find any single page that gives a detailed, complete info on how to enable PROTON aka Steam Play or SteamPlay to work, so here are the steps.

-Click on Steam → Settings
-On the left side, click on Account
-On the right side, below Beta participation
-Click on Change.
-Select Steam beta update.
-Click OK.
-It does an update and restart Steam.
-
-Go to your Steam Library.
-Any game that is Windows only, right click, click Properties
-On the left side, click Compatibility
-On the right side, checkmark Force the user of a specific Steam Play compatibility tool
-From the list, I choose Proton 5.13-5
-Click on the X on the top right corner.
-Now from the main Steam window, the Install button becomes enabled for your game.
-
-You might want to visit protondb gertataf com and see if the game you are interested in actually does run and how well it runs. There are lots of comments from other users. Also, post a review to increase the database data.
-Welcome to the club!

I could also make a manual, a PDF if you want, with screenshots.
~~~~vmelkon