I have not come across any information in-game or in online forums how to add relics to weapons and armor. I came across a video showing to press the ‘F’ key in some inventory screen. But when playing the game, I see absolutely nothing evident on how and where to do this. Can someone please give me the full steps on how to add relics? I have about seven of them that are just wasting inventory/stash space.

I found the answer:
Speak to the blacksmith Obryht in Horns. He will set you up with a miscelaneous 'Misc' quest (NOT shown as a Main or Side quest). This opens the opportunity to do a couple things: 1. Add relics to your gear and 2. Sharpen weapons/armor. You can spend the coin to do each of these to improve your gear, but this option when visiting the blackmith will not show without 'activating this miscelaneous quest.

Keep in mind:
These Misc quests (not Main/Side quests) are not completable, and will not show as 'Done' quests in your quest log(s). All of the Misc quests are simply guidances to earn currency by collecting Bandid Hides or Keeper Tags, and also visit the blackmith to add relics and perform equipment sharpening as one sees fit during the playthrough.