Runes

You craft runes in skyhold after getting an Arcanist, and you use the weapons crafting station, far right option at top filter.  You install them in the weapons upgrade station.  You find full runes in the game depending on options you make and where you go.  You can find the crafting schematic in areas where there are veilfire lamps, and you usually have to light the veilfire, then get a veilfire torch and carry it to expose the rune to activating.  Activate the rune and you then get the option to be able to craft it, if you have the necessary resources.  Rune "bases" can be found from a merchant in skyhold courtyard, and depending on the rune, it will take different items depending on what they actually do.

Easy place to find a Rune schematics, are the first Solas quest area of activating Elven Artifacts in the Eastern side of the Hinterlands(the cave with an Elven female mage fighting demons outside it), another is in the Cave at the End of Long River in The Storm Coast, and the last I can think off the top of my head is in the Oasis, inside the temple that is opened by shards you find, Veilfire is near the Spirit door, and the Rune schematic is near the Fire door.