multi domain/mapping

Hi, I was able to successfully setup the multisite mode for the first time. I would like to allow users to create their own sites, as well as map domains. I tried to install the multi-domains plugin, but keeps saying it failed. I was able to install the domain mapping plugin and I think I made the right changes to the sunrise file, but have some questions. (based on this link https://premium.wpmudev.org/blog/how-to-build-a-wordpress-multisite-network-with-multiple-domains/)

1. How do you create the user interface that would allow a visitor to view option to create a site?

2. Any advice on getting the multi-domain plugin to install?

3. How can domain mapping be automated (or synced with an api?) so users have the option of searching for a domain, buying it, and having the dns mapped right to their site (vs them having to go in to registrar to manually change dns settings.) Similar to wordpress.com

Thanks!