Click the gear icon on CS2s library page and select the beta "csgo_legacy" in its Steam properties. Wait for the update to finish.
Click the add non-steam game button in the bottom left corner of your Steam client and click "Browse" to open the file explorer.
Navigate to wherever you installed CS2, usually in C:\Program Files(x86)\Steam\steamapps\common\, and select the csgo.exe there.
Click the button to add the selected programs to Steam, then on the newly created library page, open the properties of the shortcut.
Enter the following text into the start parameters for the shortcut.
Open the folder in which CS:GO is installed again. This time you can use CS2's library page to get there right away.
Open the steam.inf file within the "csgo" folder. Edit the version numbers like on the screenshot below.
Download the CURRENT and PATCH releases from ModDB. You also need to install WinRAR if you don't have it and use Windows 10.
After extracting both archives, paste the cscomod folder from them into your CS:GO installation directory (where the csgo.exe is). Do the CURRENT first, then the PATCH.
Start the game from your Steam shortcut, click the Play button on the left and select "CS2-Serverbrowser" or "- Choose another option" from the dropdown menu.
Add 193.187.255.145 to your Favorites tab. You need to restart your game before you can connect for the first time.
If your game gets stuck on downloading a map, load that map up in Training with Bots mode before you connect to the server.