Adding custom domain to subsite on multisite installation

Hi,
I have installed domain mapping plugin, have made the changes to the wp-config file to enable the plugin to work.

Could you please provide guidance on the next steps.

Went to dashboard of the subsite, then
Tools / Domain Mapping.

Could you please give guidance on the steps from here.
The site was under http://www.mainsite/subsite
I had a number of pages and products already (a separate issue, will all products and pages automatically be given new structure of http://www.customdomain.com.au/page (product/post etc)

I did try to type in the new custom domain name then click the map domain button, it comes u with an error saying that the Domain Name 'is unavailable to access'

The error also advises to check DNS records are configured correctly. What would be necessary to check here.

Thanks