Advanced: my DNS is elsewhere / a website already runs there

If the domain already has a website or email on another service and you want its DNS to stay there, use the "Advanced" option at the bottom of the screen instead of nameservers: you add two records (an A record for your domain's root and a CNAME for "www") at your current DNS provider, and all your other records (including email) stay untouched. Both values are copyable on the screen.