I don't have a lot of experience with PBEM, so I don't have a lot of advice there. Teaching new players, however, is something we're adding to the text.
The chapter is still a work in progress, but here's some of the top hints:
-Before you explain anything about rolling, moves, damage, etc. tell them the basic flow of play: You say what your character does, I describe the results. Sometimes the rules will step in to guide the action, sometimes they won't. Your job is to play your characters, say what they say, think, and do. The rules will kick in as needed.
-Give them a quick rundown of how moves work. Roll, + something, 10+, 7-9, 6-, etc.
-Give them a high level idea of how damage, HP, armor, and load work. This will mostly come out in play, and they probably already have the idea.
-Explain that XP is their responsibility, not yours. When they roll the right thing, they get XP. When they fulfill their alignment, they get XP. If they're not sure if they've fulfilled their alignment, just ask, that conversation is usually really good.
That's the high level of the things I would tell experienced D&D-style players about DW to get started. Hope that helps. If you find them asking about stuff that I didn't mention here, I'd love the hear about it.