If you haven’t read up about the Netherlight Crucible, wowhead’s guide is actually a pretty good reference. I found it confusing at first, but once I unlocked it and could do what they’re talking about, it made more sense.

If you want to get right to it, tl;dr, after you finish the quest and get the achievement:

1. In the Vindicaar, click on the Netherlight Crucible
2. Click the first row icon for each of your artifact’s traits, each one gives your artifact +5 ilvls.
3. This unlocks the “shadow” and “light” paths for the second tier. You have to have a certain number of traits in each class specialization’s artifact to select these.
4. Once selected, shadow or light are locked in for tier 2.
5. Tier 3 are additional traits for the relic and which one you can select depends on whether you went shadow or light in tier 2.

Some things to note:

Use the preview box at the top to soulbind a relic in your bag, and find out what tier 2 and tier 3 traits it can have. This locks it in, good for all those relics you were saving.

Raidbots can sim relics with Netherlight Crucible. Make sure you check the box for “crucible row 1” after you’ve gotten the +5 ilvls trait for each one (Netherlight foundation). The “Crucible Row 2” drop down has all the “shadow” and “light” traits available. Select the one that you can pick for your relics according to the tree in game. Crucible row 3 has all the artifact traits available to your specialization. Select the ones you have available for each tree of the three for that row.