1) Log in (from the home page, or under Account on the left bar of the map viewer)
2) Click "New Map" under account
3) Enter a name and click "Create Map"
Then you can draw lines and waypoints on the map and save them as a .GPX file or send them directly to a Garmin GPS using a browser plugin that Garmin makes.
I realize this is a little backwards for most people, and that it makes more sense to be able to start drawing on the map and then choose to save it. Fixing that is on my to-do list, but there are some other features I'm trying to get done first.