How to become admin on a Rust server

From the GamePanel console, type ownerid followed by your SteamID64, then server.writecfg. Reconnect to the server for the rank to apply.

Find your SteamID64 (17 digits) on your Steam profile or a site like steamid.io.
GamePanelConsole: ownerid 7656119XXXXXXXXXX "YourNickname", then server.writecfg.
GamePanel console: ownerid then server.writecfg, the server replies Added owner and Config Saved
Disconnect and reconnect in-game.
For a moderator (fewer rights): moderatorid followed by their SteamID64, then server.writecfg. To remove a rank: removeowner or removemoderator followed by the SteamID64, then server.writecfg.

Common errors

Rank lost after a restart: server.writecfg was not typed after ownerid.
No effect: the number entered is not a SteamID64 (17 digits starting with 7656119) but a friend code or the custom name in the profile address.
Admin commands refused in-game: you have not reconnected since the rank was added.

Admin commands are then typed in the game console (F1 key).