After following the setup guide to get LIV avatars working, open the LIV App and hit Launch Compositor. Click on the MANUAL tab.
Launch the game you want to play from the SteamVR UI in VR or the Steam desktop app. If using OBS, you will want to capture the game using Game Capture or OpenVR Capture. Capture the LIV App Output using Game Capture also. Next, open the properties for the LIV App Output and check the Allow Transparency box.
In OBS, you will want to make sure the LIV APP Output is listed above the game capture, as seen in the following screenshot.