Domains
Point a domain at a VPS
A and AAAA records, nameservers, and TLS on the server.
If Steady hosts DNS
Keep our nameservers. Create an A record for @ and www (or CNAME www to @) pointing at the VPS IPv4. Add AAAA only if the VPS has IPv6 and nginx/caddy listens on it.
If DNS is elsewhere
Leave nameservers on that other host. Create the same A/AAAA records there. Steady’s DNS editor will not be live.
TLS
Certificates are issued on the VPS (for example Let’s Encrypt) once the hostname already points at the server. The domain product’s SSL for website subscriptions is not automatically the certificate on your self-managed VPS.