In short: install the app, enter your Tvheadend server address and sign in. Done.
You need a running Tvheadend server with tuners and channels configured. The HTSP interface (default port 9982) and HTTP (9981) for picon logos are recommended.
Download the latest package (APK) from the releases section on GitHub and install it on a phone or Android TV / box. On a box, allow installation from unknown sources if the system asks.
After launch, enter the server address (e.g. http://192.168.1.10:9981) and a username and password if the server has login enabled. The app verifies the connection.
Open Channels for live broadcast, TV guide for the EPG, Archive for recordings and catch-up, or Radio. On a box you control it all with the remote.