EDIT 2: Since October 13th (thank you bigjoe), you can select workshop maps in TF2 when writing in console easily and in the local server browser. Thank you all for the support and hurrah for Valve adding a cool feature!

bigjoe says: "Since today's update, you don't need all these steps anymore. Just use "maps *" command to see the workshop map names, then type "map wo" to start seeing workshop maps to auto-complete their numbers with up and down arrow keys, or Tab and Shift + Tab (that last one won't work unless you changed the Steam overlay shortcut keys)."

EDIT: A better guide has surfaced, and it requires no copying!
Click here to see that guide!
Feel free to continue with this guide if you really want to.

For example purposes I chose the Abandoned Turbine map.

But you can choose any map you want...
Move onto step 2 after you have subscribed AND downloaded the map.

First up, take a quick travel to the workshop folder.
For Linux users, it's "~/.local/share/Steam/steamapps/workshop/content/440/" (thank you Kira!)
For Windows users, it's "X:\Program Files\Steam\steamapps\workshop\content\440"
X represents your main hard-drive, normally being C:\
For x86 based systems, change "Program Files" to "Program Files(x86)"








You need to look for the map you want: if it was the last map you downloaded, look for the folder with the last modified date.
If not, just look around, you'll find it in one of the folders you have.
Depending on what the map was and how many you've downloaded, there might be more or less folders for you, with different numbers.



There we go!
Next up you'll want to copy it:


Go over to X:\Program Files\Steam\steamapps\common\Team Fortress 2\tf\maps and paste it!


That's basicly it! It's now 100% on your TF2.

* to run the map you need the developer console, please check another guide for that

To run a map, you need to go to the developer console in TF2 and type: map (name of map).
You don't need to include the .bsp part.

That's all.