How to run startx as non-root?

First a clarification, X is not a window manager, it is a windowing system.

Now, the ~/.Xauthority file is simply where the identification credentials for the current user's Xsession are stored, it is the file read when the system needs to determine if you have the right to use the current X session.

You should never copy an existing one from another account, the file should always belong to the user running X and is created automatically when you start a new X session. So, just delete the one you have, and then run startx again, everything should work as normal:

$ rm ~/.Xauthority; startx

I think an issue is that people login as root, then switch users using su -l username, which creates/spawns a subprocess under root owned by "username". X will use the .Xauthority of the top process (in this case root), which "username" does not have permission to use.

The error will be shown:

Cannot open virtual console 1 (Permission denied).

To solve this, just run login instead of su to start a new shell as a top process.