On our Cloud, it's a pretty simple process. The first step is obviously, you need to purchase your domain. Then you can go to Client Area -> My Purchases -> click your package -> Change Domain. Insert your domain there, it will give you the nameservers to change at your domain registrar (where you bought your domain). Our team will receive the request and should the nameservers be set, we will make the change so your community loads over your domain.
If you have questions on how to set your nameservers, contacting your domain registrar is the best step as they all can be slightly different 🙂 .
If you want to use a sub-domain, the process is slightly different but sounds like you want to use a root domain so we won't go into that process 🙂 .