Unable to establish external trust

I'm a lot new to Windows Server 2003, Trusts, etc., and have some questions.

We have a Windows 2003 Server which our Domain Controller, however our DNS is handled by another server in the forest. We are, I think the correct term is "a sub-domain" of that dns server.

I need to allow access to certain folders on our server to someone that is connected to us via fiber, but not part of our network. We also have a firewall set up between us.

The network administrator from this other location and I have been trying to get a trust established between the two of us, but it's simply not working. We get errors such as RPC server not available, netlogon errors and such.

Since I'm new to all of this networking in 2003 I would like to understand how this scenario would work. We have WINS set up on the dns server for them and also forward lookup zones are set up in dns? But for some reason no matter what type of trust we try to establish we're getting the errors. I'm not sure if there is a dns or wins issue on our side or theirs. But does anyone have like a step by step guide to setting up a trust between a sub-domain of a 2003 forest and a domain of another forest? I apologize if my terminology isn't correct, but like I said I'm pretty new at this. Thanks!