A Record/Main Domain versus Subdomains

I’m not getting how A records/hosting with Dreamhost works, and looking for some help in pointing me in the right direction as to what I need to do…

So, have a Wordpress install on a fully hosted domain/site. I now want to have that Wordpress install accessed via a subdomain only, while the main domain points to a non-Dreamhost server.

Using A records, I’ve got the main domain pointing to the non-Dreamhost server when www isn’t used, but I can’t seem to get the A records to work for the subdomain (pointed the Dreamhost IP for the main domain).

I’m hoping to avoid having to move the Wordpress install just to use a subdomain if possible.