There are no guides/tutorials for this yet, as it’s a pretty fresh territory here.
What Vikas said is absolutely correct. I would reconfigure your current DO Ghost installation to run under a subdomain of the domain you’re planning to use. Here’s a topic on how to do that, but you’ll also find it in the Docs.
If you’d like to use https://example.com change the URL of your DO installation to be https://content.example.com or something like that. This way, your Content API URL will be https://content.example.com and you can access Ghost Admin with https://content.example.com/ghost
The you’ll point https://example.com to your Netlify site and you should be sorted.