Question

how to write a formal letter

Answer 1

Use correct grammar and appropriate sentences. 
For example:

Dear Mr.Smith,

My wife bought a china cabinet from your company store in Vienna, Virginia. We had 2 young men, who did not speak good English, assemble our cabinet. They left a short while after. I was assembling items in the cabinet when I realized it was missing 10 nails and a light. I called your store and they did not pick up. I called in a few hours later. A person picked up and told me people would come and help me on Monday, March 8th at 12:00. No one came at 12, 1, 2, 3, or 4! 

I called your store and asked why. They did not know and rescheduled it, for the next day at 3. No one showed up at 3, 4, 5, 6, or 7! I want someone to come or my money back. Thank you. My number is 777-888-9990.
Sincerely,

Mr. and Mrs.Harbor

Answer 2

In English there are a number of conventions that should be used when writing a formal or business letter. Furthermore, you try to write as simply and as clearly as possible, and not to make the letter longer than necessary. Remember not to use informal language like contractions.

Addresses:

1) Your Address
The return address should be written in the top right-hand corner of the letter.2) The Address of the person you are writing to
The inside address should be written on the left, starting below your address.

Date:

Different people put the date on different sides of the page. You can write this on the right or the left on the line after the address you are writing to. Write the month as a word.

Salutation or greeting:

1) Dear Sir or Madam,
If you do not know the name of the person you are writing to, use this. It is always advisable to try to find out a name.2) Dear Mr Jenkins,
If you know the name, use the title (Mr, Mrs, Miss or Ms, Dr, etc.) and the surname only. If you are writing to a woman and do not know if she uses Mrs or Miss, you can use Ms, which is for married and single women.

Ending a letter:

1) Yours Faithfully
If you do not know the name of the person, end the letter this way.2) Yours Sincerely
If you know the name of the person, end the letter this way.3) Your signature
Sign your name, then print it underneath the signature. If you think the person you are writing to might not know whether you are male of female, put you title in brackets after your name.