Adding custom domain to subsite on multisite installation

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 (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.