Discretion first.Search before you resign. We never introduce you to your current employer and name you only after your approval.
Check deadlines.Read the contract: notice period, date (15th/end of month), form (written, signed, not a scan).
Sign the new contract first, then resign.Never the other way round.
Keep the resignation short and friendly.One paragraph, date, thanks. No reason needed.
Reference (Zeugnis)— you are entitled to a qualified reference; a grade of "good" or better is standard for proper performance. Secure an interim reference before you leave.
Remaining holiday, overtime, bonus— clarify, have it confirmed in writing, watch the cut-off dates.
Handover— document it cleanly. That is your reputation.
For physicians— specialist training reference (Weiterbildungszeugnis) from your head of department, logbook up to date, inform the medical chamber if you change chambers.