Thanks to a couple of forum posts and other documentation (see below) I’ve been able to piece together the following process.

Starting at the DreamHost end of things go to your control panel <https://panel.dreamhost.com/> and navigate to ‘Manage Domains’.

Click on ‘Add New Domain / Sub-Domain’ and add your primary domain, e.g. example.com, as fully hosted making sure to choose ‘Remove WWW’ where it ask: ‘Do you want the www in your URL?’ Don’t worry this should not effect your current web site/hosting if your domain is not hosted by DreamHost. We just need to do this so that when we can add the subdomain.

Return to the Domain Listing (main screen of ‘Manage Domains’) and next to where your primary domain is listed in the ‘Web Hosting’ column click the Remove button (blue box with a red x to the left of the word Remove). This removes the Web Hosting settings from the primary domain and set it to DNS Only.

or

Click on ‘Add New Domain / Sub-Domain’ and add your primary domain under the ‘DNS Only’ section.*

Now you should be able to go to ‘Add New Domain / Sub-Domain’ and add your subdomain, e.g. sub.example.com, as fully hosted.

Once you have done this return again to the Domain Listing and click on the DNS link under your subdomain. Towards the very bottom of the page you’ll find the section entitled ‘Non-editable DreamHost DNS records for [sub.example.com]:’ where sub.example.com is your subdomain. Under here find the IP Address (e.g. 173.236.###.##) for the A record next to the root of your subdomain, e.g. if your subdomain is sub.example.com the root is ‘sub’. Copy the IP Address down somewhere.

Now go to your domain host and where you configure your DNS entries (can’t be more explicit than that as every domain host is different) add an A DNS record type to point your subdomain at the IP Address you got from DreamHost earlier.

e.g. Host: sub.example.com, Record type: A, Value: 173.236.###.##

You may have to wait anything up to 48 hours for these DNS configuration changes to filter through the inter-webs so don’t panic until that time has passed. If you have problems with this please contact DreamHost or domain host not me. I’m only providing this info here because I could not find a simple description elsewhere and I thought it might prove useful.

* I’ve not tried this, but it should work. The articles I reviewed suggested doing it the first way, which I did and it worked for me, but they gave no reason for doing it that way.

• Pointing subdomain to DreamHost] – DreamHost Forum
• Move WordPress to DreamHost – wiki.dreamhost.com