1. Writing a job application (cover) letter
The main parts of an application letter are:
1) Defining Objectives, 2) Planning, 3) Writing a draft, 4) Evaluating the situation, and 5) Revising it for perfection.
Define your objectives
The objective on an application letter is not the same thing as the objective statement on your resume. Think of a letter as a substitute for an interview. In the application letter, the employers, looks for things that they couldn’t find in your resume. In particular, they may look for:
Your responsibilities
How do you look like as an employee or as an individual?
Your reasons for applying to their organizations.
Why do you want to work for that company?
Your understanding of how your education and experiences relate to the job you are applying.
How will you fit in the company? What do you have that the company is looking for?

2. Writing a job application (cover) letter Planning
In planning your application letter, try to prepare your letter to have three parts: Introduction, Body, and Conclusion.
Introduction
Write why your are applying for the job and say something about yourself, where you go to school, what your major is, and when suitable, talk about date of graduation.
This is a place you want to say why you have selected this company. So, before you write the letter, look for some information about the company and the job and refer to them as precise as possible in your introduction. Basically, you want to tell the employer “I know you well enough to say that I want to work for you.”
It used to be a good thing to say “good things” about a company and its services. Forget it Instead, tell them what specific things they do that is attractive to you.

3. Planning
Body
By now, you have read and understood what the employer is looking for. So, here is your chance … hit it.
Tell the employer how well your qualifications fits to the job that they have.
Do not just copy things from your resume.
Start by highlighting some of the things you have done and find a way to connect them to the job they have.
Talk about the particular skills that the job requires and make a connection with the courses that you have taken or experiences that you have had.
Conclusions
Make sure to tell the employer one more time that you feel you are a fit. Show that you are anxious to hear from them. Tell them how you are planning to follow your letter (will call in week or will write in a week to find out). And thank them for considering you.

4. Writing the draft
Perhaps this is the hardest part of all. Even after you plan everything, you may still have difficulty drafting the letter. For many there will be some questions:
What tone would be the best?
Where is the line between confident and conceit?
Where is the line between determination and pushiness?
In any case, you want to convey to the employer that you are self-assured but not overconfident. As a rule of thump: DO NOT USE the tone “I am sure”.
GOOD: Something like: I hope you find that my education and experiences qualify me for this position. Or something like that.
Remember that the right tone in your conclusion is as important. Do not make a conclusion on yourself.
BAD: I would like to meet you at your earliest time.
GOOD: I look forward to hearing from you to further discuss my qualifications with you.

5. Evaluating the situation
Once you draft your letter, read the letter to yourself several times and make sure it is delivering the message you wish to get across.
If you are asking someone to review the letter, tell him/her to also look into it as if he/she is the employer and to give you their comments.
Do you need a clue, I would ask my sister in-law to read my letter. I knew, if she would hire me, then I had a chance with others.
When you evaluate, make sure that your tone and the words are clear to understand.
Revising for perfection
You may have found some problems with your draft, but no know how to solve the problems. That is true with your reviewers comments too. They may tell you what the problem is, but not how to fix it.
Simply, think what the reaction would be when the employer reads your letter and make sure to use SIMPLE but CLEAR language.