In short
Whether an expired domain can be registered depends on its status at the registry: if it is already free, it is registered regularly; if it is still in a protective/quarantine period, only after that ends — then a backorder helps. Whether the registration is worthwhile is decided by prior vetting.
Two cases
- Already free: regular registration via a registrar.
- Still in a protective period: wait for the re-release; a backorder targets it. The timing depends on the TLD (e.g. 30-day RGP for .de).
After registration
Set up DNS/forwarding (nameservers & DNS), plan a project or park the domain. With existing history, set up the reactivation cleanly.