It's fairly straightforward nowadays and will get even easier this year.

In KDE, you can just enable HDR and hit apply. There's also a calibration tool integrated that is a little bit barebones but it does the job.

For gaming, you currently still need Proton-GE until Valve's Proton ships with the necessary libraries. You can easily download them using ProtonUp-Qt.

Once that is done:

• Restart Steam
• Right click the game -> Properties...
• Set the launch options to PROTON_ENABLE_WAYLAND=1 PROTON_ENABLE_HDR=1 %command%
• Under Compatibility set the checkmark "Force the use of a specific Steam Play compatibility tool" and select the Proton-GE version you just installed before.

That's it, you can now enable HDR ingame.