I never used Crazy Domains, but the process is same for every domain register (GoDaddy, NameCheap, Crazy Domains).
One of the possible ways is to point name servers records to DigitalOcean and then manage domain using DO Cloud Control Panel.
This tutorial explains how to point name servers, but it doesn't show for Crazy Domains. However, you should get what you need to find there.
Edit: There's How to change nameservers in their docs.
by Josh Barnett
by Kathleen Juell
DNS is the naming system that is used to bind a domain name to a web server. In this guide, we will change the nameservers for a domain that we have registered and point it to a DigitalOcean Droplet.
