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Keep Steam open in the background (use a secondary "burner" account to be safe).

Assuming you have the base game (v1.09 is the latest as of this article) and the Online-Fix.

Ensure Raft.exe is allowed through your Windows Firewall. Public and Private networks should both be checked.

When you launch the game via the Online-Fix, Steam will automatically add a game called "Spacewar" to your library (usually hidden). This is the dummy game hosting your connection.