Use the block formal. In this business letter style, all text starts flush-left. Instead of indenting, leave a blank line between each part of the letter: return address, date, inside address, salutation, body of the letter and closing.
Print your letter on letterhead, if possible. Letterhead helps create a good impression at VFS as it lends credibility to your visa application and indicates you have a business to return to. If you don't have letterhead, type in your return postal address so VFS knows where to send correspondence.
Skip a line and enter the date and then the inside address. The inside address is the VFS office nearest you. Check with the United States Embassy or the VFS website if you aren't sure of the address.
Use the salutation "To Whom It May Concern:". Note that the salutation is followed by a colon.
Be direct. State that you are applying for a visa to the United states and name the category. It could be to visit, study, work, join family or transit. Maintain a business-like tone at all times and stick to the facts in your letter to VFS.
List your supporting documentation in the body of your letter. Depending on the type of visa you are applying for -- use the application form as a guideline -- mention each item in your covering letter. If you are applying for a visitor's visa, make sure that a copy of your return ticket is attached. A student visa will require you to have a letter from the educational institution and bank statements to prove that you can support yourself while studying in America.
Sum up that you have met the visa requirements in the concluding paragraph of your letter. Mention that if any further information is required you will be glad to supply it. Provide VFS with your phone number and email address.
Close the letter with "Sincerely," or "Sincerely yours,". Note there is a comma after the closing. Leave three blank spaces for your signature and type your name.