Job portal Stepstone.de lists a few basic tips for writing a cover letter. Spelling and grammatical errors are a co-criterion when applying.
Stay true to yourself in your application letter, says the portal. The Federal Employment Agency has published a checklist with some important points for the application documents. It's not just about getting a job - the position has to be a good fit for you as well, the agency says. The cover letter should not be more than one A4 page.