You'll need to submit a letter for each position you apply for.
The format of the letter also conveys aspects of your personality. Therefore, it is always advisable to write it in a logical order, follow a clear structure, and ensure there are no spelling mistakes.
It isn't always necessary to send a cover letter. However, unless you're told otherwise, a cover letter is welcome because it allows you to highlight specific details from your resume and pique the interviewer's interest in getting to know you better.
The cover letter should be no longer than one page and can be sent as a document along with your resume, or it can be included in the body of the email (with the resume as the only attachment).
It should be written in a concise, clear, coherent, and formal style, following this structure:
First paragraph
Here, you should explain the purpose of your application. You should refer to the job posting and the position for which you are applying, noting that you are attaching your resume so that you may be considered for the position.
If you know which company this is, this is your chance to explain why you’re interested in the position and what motivates you to apply.
Second paragraph
This section highlights your most relevant education, skills, and professional achievements, always in relation to the specific requirements of the position you are applying for.
This is also where salary expectations are typically listed, but only if explicitly requested in the job posting.
Closing, greeting, and signature
Don't forget that your goal is to secure an interview, so it's a good idea to express your willingness to meet with the interviewer.
Types of letters
There are two main types of cover letters:
Submission in response to a notice
The job posting may appear in the newspaper or online, and it may come from the company hiring for the position or from the recruitment firm conducting the hiring process on the company's behalf.
You will need to clearly state which job posting you are applying for (job title or reference number).
In this case, you’ll know exactly what skills are required for the job, since they will have been listed in the job posting. You’ll need to list and describe your knowledge, skills, or experience in those areas, aiming to pique the reader’s interest.
Walk-in
In this case, it’s a good idea to have a list of companies you’d like to work for on hand. Find out who will be receiving your resume—whether it’s the company’s director or the Human Resources department—so you can address the cover letter to them.
In your cover letter, you should specify which position you are applying for in case a vacancy arises, explain your motivation and your specific interest in joining the company, and state why you believe you are qualified to work there.