Download and install Steam client: https://store.steampowered.com/about.

Launch the Steam client and login to your Steam account (or create a new account if you don't have).

Click on "ADD A GAME" in the bottom-left corner of the client.

Choose "Activate a Product on Steam…".

Follow the onscreen instructions to complete the process.

After successful activation, your game will be visible in your Library.

System requirements

Minimum:

Requires a 64-bit processor and operating system
OS: WINDOWS® 7, 8, 8.1, 10 (64-bit required)
Processor: Intel® Core™ i5-4460, 3.20GHz or AMD FX™-6300
Memory: 8 GB RAM
Graphics: NVIDIA® GeForce® GTX 760 or AMD Radeon™ R7 260x (VRAM 2GB)
DirectX: Version 11
Network: Broadband Internet connection
Storage: 48 GB available space
Sound Card: DirectSound (DirectX® 9.0c)
Additional Notes: 1080p/30fps when graphics settings are set to "Low"

Recommended:

Requires a 64-bit processor and operating system
OS: WINDOWS® 7, 8, 8.1, 10 (64-bit required)
Processor: Intel® Core™ i7 3770 3.4GHz or Intel® Core™ i3 8350 4GHz or AMD Ryzen™ 5 1500X
Memory: 8 GB RAM
Graphics: NVIDIA® GeForce® GTX 1060 (VRAM 3GB) or AMD Radeon™ RX 570 (VRAM 4GB)
DirectX: Version 11
Network: Broadband Internet connection
Storage: 48 GB available space
Sound Card: DirectSound (DirectX® 9.0c or better)
Additional Notes: 1080p/30fps when graphics settings are set to "High"