Proton is the Valve's compatibility layer for running Windows games on Linux through Steam.
This repository contains recepies for building Proton from source on top of freedesktop-sdk in flatpak format, intended for running from the Steam flatpak.
This unofficial build isn't supported by Valve and wasn't tested with all possible games and cases. It can behave differently from official builds. Use at your own risk.
First add Flathub repository and install Steam from there, if not already. Then run
flatpak install com.valvesoftware.Steam.CompatibilityTool.ProtonLaunch Steam flatpak and select "Proton (flatpak)" compatibility tool from drop-down list.