• Start Valheim from Steam
• Start Game
• Select character
• Click Start
• Click Join Game tab
• Click Join IP button
• Click the ip:port text entry field
• Enter valheim.c9h.org into the field and press enter
• Click the join/start/go button
• when prompted to Enter Password, enter the super secret password. Then press enter.
• Wait
• ???
• profit

Consider using Valheim Plus There is some documentation on how to install the module, if you decide to use it. The server has it enabled.