You'll need to first install the WireGuard app from the Google Play Store. Then, download the "Wireguard Config" file for each user on the desired device, or download and send it to them over any secure messaging app or channel.
Once WireGuard is installed, tap the blue action button and select either "Create from file or archive", if you have downloaded or have been sent a config file, or "Create from QR Code" to use the camera to scan the QR Code directly from TunnelHero using the camera.
If you enabled "Connect On Demand" the VPN will connect automatically whenever it is able. Most users will want "Connect On Demand", but while it is enabled then simply turning off the VPN will not cause it to stay disabled; it will just "Connect On Demand" again. To completely turn off the VPN you'll need to disable "Connect On Demand".
On Android, you can turn off "Connect On Demand" by opening the Settings app, tapping on Network & Internet, tapping Advanced, then tapping on VPN. WireGuard will be listed on this screen. Tap the gear icon, then uncheck "Always-on VPN".
On this screen you can also configure "Block connections without VPN" for additional security and peace-of-mind, if "Always-on VPN" is enabled.