Just in case you found this guide and wanted to learn how to get a map from the workshop in the first place, simply go to TF2's game page, and scroll down.
It should look something like this.
(Unless you don't have premium, in which case DLC won't appear for you.)


Click the button that reads "Browse The Workshop"


And you should be taken to a page that looks something like this.


If you hover your cursor over "Browse", a dropdown menu should appear and "Maps" should be an option. Click on that, and you'll be taken to a page like this.


From there, find a map you like and click on it. For the purposes of this guide, I'll install Abandoned Upward.

Click the "Subscribe Button", allow it to download, then continue to the next part of this guide.

So, you've got a map that you've subscribed to and want to play on it.
First, you'll need to locate your Steam Workshop folder. (Mine is in C:\Program Files (x86)\Steam\steamapps\workshop)
I'm afraid I can't help you during that step, as I don't know how your computer works.

You'll want to open up the "content" folder, then open "440" within that.
You'll see a folder (or folders, depending on how many you subscribed to.) with seemingly random strings of numbers for names.


Inside those folders are your map files, so you'll want to copy those for the next step.

So, you have your map file copied and are ready to install! Now you'll need to locate your Team Fortress 2 folder. (Mine is at C:\Program Files (x86)\Steam\steamapps\common\Team Fortress 2)

Open that up and you'll be greeted with a lot of folders, some text files, possibly an image or two, and hl2.exe.
From here it's fairly straightforward.
Open the tf folder, then open the maps folder.
From here, it's as easy as pasting the map file you copied!

Now that the map file is properly copied, we can open up the game and try the map out!

First off, open Team Fortress 2. (If you don't have the game, then what are you doing here?)

Open the "Find a Game" Menu, scroll down, and select "Create Server".
There's a fairly simple "Map" drop down menu, in which you have to locate your map.



Start the server, and...



Enjoy!