Install the Game:

After purchase, click "Library" at the top of Steam.

Find your game in the left sidebar.

Click the game, then click the blue "Install" button.

Choose the install location (if prompted), then click Next.
Wait for the Download:

Steam will download and install the game automatically.

You can monitor progress at the bottom of the Steam window or under the "Downloads" section.

Launch the Game:

Once installed, click "Play" from the game’s page in your Library.