I've subscribed to an item on Steam Workshop - how do I download it?

Now that the Portal 2 Perpetual Testing Initiative is out, I've been looking at some of the available maps, but I'm not sure how to actually download them.

I've gone into an author's or collection's page, where I can subscribe to an item. I've clicked the Subscribe button and the subscriptions have been added successfully, but nothing else happens - no downloads begin, no download buttons appear.

How do I actually download the maps I've subscribed to? I hope it doesn't involve going into the actual Portal 2 game, as it's a bit of an inconvenience to have to start it up just to download the maps.

N.B. I've left the Portal 2 tag out because I assume all Steam workshop pages work in the same way, and answers will apply to other games as well.

You do not do anything. The content should download automatically. In the case of Portal 2, the map will download when you try to start it for the first time