How to Activate Steam Offline
Step 1
Log in to the Steam Desktop Application using the account information sent to you. (Do not log in from a browser or mobile application).
Use the Change Account option to log in, and don’t forget to check Remember Me.
Step 2
Click on the game in the Library, then go to:
Settings > Properties > "...store game files on Steam Cloud" and disable it.
(If it is already disabled, no action is required).
Step 3
Download the game.
(If the "Install" button is not visible, click the small triangle next to it and select "This Computer").
Step 4
Open the game once, then close it after one minute.
Step 5
In Steam, click on Steam in the top-left corner, then select Offline Mode.
Always play the game with Steam in offline mode.