basically you have to pressthe subscribe button

wait for steam client to completely download it

then you have to run the game

then you have to go to custom games menu

then you have to select the map

then you have to pick the settings that you want

and finally you have to press the play button

you may need to exit the game

verify game cache

unsubscribe to the map

delete the local copy

subscribe to the map again

allow it to downlod

and run the game again