Post-purchase instructions:
1. Unzip the downloaded archive.
2. Download and install the latest version of Steam Desktop Authenticator (SDA) -
https://github.com/Jessecar96/SteamDesktopAuthenticator/releases
This program is needed to receive codes from a mobile authenticator.
3. Launch SDA. In the top panel, click on File - Import account - from maFile - specify the file from the downloaded archive.
Now the SDA shows the codes for entering your account.
4. Open the text file from the archive, there is the login and password from the account.
5. Log in to the Steam application using the code from the program.
6. Click on "login" in the upper right corner of the steam program and select "account details"
7. In the "Contact Info" section, click on "Change my email address". Confirm the action in the SDA by clicking on the "View Confirmations" button. Also change the password and phone number.
You can remove the authenticator using the revocation code. And find the line "revocation code", it is indicated through a colon.
That´s all. This is your account and no one else will log into it except you. You can download the game from the library, open safes and enjoy the game.