During ordering we show the two nameservers your domain must use. Copy both, open your registrar's DNS settings for the domain, and replace the existing nameservers with these two.
DNS changes propagate — most domains resolve within a few hours, occasionally up to a day. The order page checks live and tells you when the domain already points here, so you know the moment it lands.
If the lookup is ever unavailable while ordering, go ahead anyway: provisioning prepares the nameservers and your panel page shows them again after the deploy.