After you've created your first watchlist (WL), you can edit it or create another one. Depending on your subscription, there may be a limit to the number of watchlists you can create.
On the Watchlist tab, when you click on the WL name, a list of all your WLs will appear. You can either select another WL or create a new one. To create a new one, just click on the search box. There will be a list of popular topics, but if you want something specific, just start typing. Once you are finished finding topics, press Go, and a new watchlist will be created for you.
Example of a partially-built watchlist
You can even add complex topics or search queries to any watchlist. For example, you might want to add Apple AND Samsung AND NOT patents to get news that talks about both Apple and Samsung but excludes any stories mentioning patent battles. This is a major upgrade in capabilities from traditional watchlists.
You can see a little more explanation on this page.