Originally posted by burn:

add your SteamID's (yours and your friends) to the admin list of the server.

you and your friends install this mod:
https://valheim.thunderstore.io/package/JereKuusela/Server_devcommands/

optionally install the mod to the server as well to enable more commands/features.

Does the devcommands mod require a mod manager then? i'm not seeing any way to install