its quite simple...download the sdk, download the game, launch the game and wait for the painfully slow load time you get on an old laptop, close the game once the menu loads, launch the sdk, select the game and start mapping. You may also want to set steam to offline mode after all this so you can stay logged in on your primary computer while still being able to map.
it also doesn't hurt to lower some of the settings on hammer so you don't have to deal with the lag on more detailed maps