7 essential principles
Cover letter explained:
The cover letter is about story-telling
The cover letter must not be the recitation of your resume
Technical skills / certifications / accreditations listed in resume can be used in cover letter only when it is appropriate
3-4 crisp paragraphs
Start with a strong statement about yourself, about your alignment with the company, its values or vision
The next two paragraphs (paragraph #2 and #3) is about specific qualities and aptitudes that matter for this specific organization and position with a proof of why you are the right candidate with relevant qualities, explain how you have developed and demonstrated those aptitudes in your previous experiences
Last paragraph is the closure, is the expression of your interest of being of value to the organization, its team and business prospects
Important: each cover letter just as your resume shall be tailored for each job opportunity - there shall be no doubt for recruiter or hiring manager that it is a generic document you have sent out to dozens of organizations.