What is needed for transfer?
Most transfers require an AuthInfo/EPP code, access to the current registrar panel, correct registrant data and a decision on DNS timing.
How to avoid downtime
Before transferring, check DNS records, e-mail, SSL, renewal date and transfer locks. A domain transfer does not always require moving hosting.
Transfer with migration
When a website or e-mail is moved as well, the order matters: backups, tests, DNS, mail, SSL and final traffic cutover.