Sure, we've been talking about Legion for a while.
Exactly like you did before.
According to Blizzard's pre-order page for the Legion expansion, there are two requirements to make a new Demon Hunter.
If you've been playing during the Burning Legion's invasion of Azeroth, you've seen that Khadgar has a quest that leads to a pivotal moment in Dalaran.
In terms level, it took me to level 100 before I was finished with it — so when you wrap up you'll be ready to jump right into the Broken Isles. In terms of time, it will take few hours if you're a leisurely sort of player like I was.
Fortunately, your new Demon Hunter will only have two talents — level 99 and level 100 tiers — so you'll only have a couple of choices, depending on your specialization (Havoc or Vengeance).
Since you're going to have to have at least a level 70 character on any realm you roll a Demon Hunter, and they're an unparalleled mobility class, I'd suggest going double-gathering and feeding the mats to your other characters to craft things.
Artifact Knowledge is a hidden currency gained by researching it in your Class Order Hall. You get the quest at level 110, and collecting Artifact Knowledge will help you speedily get your off-spec Artifacts maxxed out. Artifact Knowledge increases how much Artifact Power you get from any items that grant it, and each level of Artifact Knowledge gives you a percentage-based modifier to your Artifact Power.