VPN for client use

I have seen advertising on television here in Australia for Nord VPN.

What are the steps involved in setting up a VPN for resale to clients on a monthly access plan? Is it difficult to do? Can it be done using Cloudmin/Webmin/Virtualmin?

My assumption is that there would be some components to this...please add any I would need but haven't got in list below 1. the proxy 2. an app on client pc 3. a means of managing client accounts (could this be done using whmcs?)