You need a couple of things for a legendary:
- You need to unlock the Runecarver first, which is basically following the storyline until you get there.
- You need a Memory for the Runecarver, which will give the power of your legendary. For monk, this Wowhead link shows the available powers and their drop locations:
https://www.wowhead.com/guides/legendary-recipes-drop-locations-sources-shadowlands-memory-of-the-runecarver#monk - You need a base item for the appropriate slot and armor type, created by crafters. Both the recipe for the item, and the materials, are new materials coming in Shadowlands, you can’t stock up on them in advance.
- Two missives, which give secondary stats, crafted by Scribes.
- Enough Soul Ash, which you get from running Torghast.
For more information, see wowhead again: https://www.wowhead.com/guides/legendary-armor-crafting-torghast-items-shadowlands