There are a few different ways you can write the subject of a letter, depending on the format you are using.
If you are using a standard business letter format, you would write the subject of the letter above the recipient's address, on the left hand side.
If you are using a friendly letter format, you can either write the subject above the recipient's address, on the left hand side, or you can write it after the salutation (i.e. "Dear so-and-so,").
Here is a link to a template for a standard business letter: https://www.thebalancecareers.com/business-letter-format-2060512
And here is a link to a template for a friendly letter: https://www.thebalancecareers.com/how-to-write-a-letter-2062780