The most useful activity when preparing to customize a cover letter is to review the company website and review the job description.
A professional cover letter is intended to more clearly explain a job applicant's qualifications that are not as explicit in the resume.
This is a strategy that can be used to make a good impression on the recruiter and to contain why you are suitable for filling a vacancy at a particular company.
So to prepare to write a cover letter you need to:
Review the company's website.
Review the job description.
That will be useful to understand the set of values and practices of a position and organization, to customize your cover letter and have more chances to advance in the selection process.
