It is very simple:
 
Through a Registrar:

In addition to offering competitive prices, they facilitate the registration process and offer several benefits and value-added services.

See the list of accredited Registrars.
At Red.es we actively promote the figure of the accredited registrar. They offer additional services at the best prices.
 
Directly through Dominios ".es":
Direct registration is only available for very specific situations that do not require intermediaries due to special circumstances. Direct registration through Red.es only provides the domain name (no additional services) and, because of the special conditions, comes at a higher price regulated by internal instruction. At Red.es we strongly recommend that you contact our accredited registrars.
 
If you have not registered, access here.
 
To manage your domain, you can access the system:
       1- Entering your identifier and password, which were assigned when you registered through Dominios.es.
       2- Authenticate yourself with your DNIe/certificate, having previously associated your DNIe/certificate with your identifier.
 
".es", ".com.es", ".nom.es" and ".org.es" domains are assigned immediately and automatically once payment has been made.
".gob.es" and ".edu.es" domains are subject to prior verification, so registration is not immediate.