irvshapiro had a really useful question/answer on Ghost.io Custom Domain and Network Solutions.
However, the solution means that your site uses URL forwarding to a named sub-domain rather than the www subdomain or site root, which is less than ideal if you want your site to be accessible like this
https://example.com
https://www.example.com
Thankfully the lovely people from support have provided an alternative approach, which has worked brilliantly for me and that was to use Cloudflare to act as the domain DNS.
That means you can implement root and www subdomain mapping whilst still using Network Solutions as the domain registrar, and it’s free.
The instructions on how to do this can be found here cloudflare custom domain setup and you might find the following video on moving from Network Solutions DNS to Cloudflare really useful.
I hope someone finds this useful, if like me they’re stuck setting up a custom domain with a Network Solutions provided domain.