I've explained hundreds of different games over the years and top-down is always the way to go:

• "This is the theme and how to win" ↓
• "These are the things you need to win and these are the things that will hinder you" ↓
• "This is how each round is structured so you can get/avoid those things".

If you do it bottom-up then people ask, "Why would I want that?" or "What's that for?" and you're constantly replying, "I'll get to that".

Practice rounds are one of my favorite ways to do it, but sadly my current group doesn't like them. Definitely the way to go if you can, though.--