[Plugin: WordPress MU Domain Mapping] Help with CNAME records

I’m having real trouble getting theis plugin to work. I have various wordpress isntallations spread across different domains and subdomains, and want to move them all into a single WPMU install.

I want to have http://blog.myapp.com mapped to http://myapp.mywpmufarm.com

So far I have:

1. Installed and activated the plugin
2. Created a blog at http://myapp.mywpmufarm.com – this is working and displaying the default post.
3. In the DNS for myapp.com, created “blog CNAME myapp.mywpmufarm.com”
4. In Tools > Domain Mapping, added http://blog.myapp.com

In the Blog settings for the ‘myapp’ blog in MU, there are fields ‘Domain’, ‘Siteurl’ and ‘Home’ – which field should be changed to the visible URL of the blog (http://blog.myapp.com)?

Also, in step 3 above – should i be pointing the CNAME record at myapp.mywpmufarm.com, or just mywpmufarm.com (the MU root)?

FYI all my domains are on the same IP address.

Any help with this would be greatly appreciated, it’s driving me mad. I’ve searched high and low for a simple step-by-step guide, but no joy.

Thanks!
Iain

http://wordpress.org/extend/plugins/wordpress-mu-domain-mapping/