General Tips For Writing a Formal Letter

• Your letter should be simple and focused; make the purpose of your letter clear.
  
• The subject is optional,but is highly recommended since the letter will not be opened by the intended recipient first. It should start with either subject: or Re:
  
• The subject (if you include one) should be left-aligned for full block format, but can be either left aligned or centered for modified block format.
• Have an margin of 1 in on all sides
• Use a std font size of 10pt or 12 pt.
• It is also appropriate to use “To Whom It May Concern” when you are making an inquiry (also known as a prospecting letter or letter of interest), but don’t have details of a contact person.
• If you are writing to someone whose name you don't know, "Dear Personnel Manager (or other job title)" and "Dear Sir or Madam" are acceptable.
• Address recipient by personal title. You can address the recipient by starting with "Dear" followed by a personal title, such as Mr. or Ms. If you have the full name of the recipient of your business letter, you can enhance the formal nature of the letter by starting with "Dear" followed by a personal salutation, such as "Dear Ms. Levatson."
• When in doubt, use Ms. If you want to use a personal title ahead of a female recipient's full name, but you are unsure of her marital status, it is always best to use the title "Ms." followed by the recipient's name instead of Miss or Mrs.
  
• Single space your letter and leave a space between each paragraph.
• Use a serif font like Times New Roman.
• Left justify your letter.
• Leave a blank line after the salutation and before the closing.
• Use the modified block layout since it looks modern and stylish.

References

1. https://www.indeed.com/career-advice/career-development/business-letter-salutation
2. https://www.lexico.com/grammar/how-to-lay-out-a-letter
3. https://www.thebalancecareers.com/sample-letter-format-2063479
4. https://www.dailywritingtips.com/how-to-format-a-us-business-letter/
5. https://www.thebalancecareers.com/business-letter-layout-example-2059703
6. https://penandthepad.com/format-cover-letter-2068849.html
7. https://www.writeawriting.com/letter/formal-letter-format-rules-guide-examples/
8. https://www.letterwritingguide.com/businessletterformat.htm
9. https://letterslibrary.com/writing-tips/structure-of-letters/  --good ref