Looks like people still struggle with installing tf2 maps from workshop, so, here's a simple guide

1) subscribe to this map
2) wait for it to download

3) open your steam folder. usually here: C:\Program Files (x86)\Steam

4) open C:\Program Files (x86)\Steam\steamapps\workshop\content\440 and search for arena_3008v1

5) copy or cut the arena_3008v1 file and paste it in your tf2 map folder, usually located here C:\Program Files (x86)\Steam\steamapps\common\Team Fortress 2\tf\maps

6) run the game, open the console, write "map arena_3008v1"(NO COMAS!!!) hit enter

congatulations, you've installed a TF2 map from the workshop