Write a letter to a bank that made mistakes on my checks while keeping goodwill.

http://owl.english.purdue.edu/handouts/pw/index.html#sub6
On this webpage, be sure to go into the first two links under Letters, Memos, and Reports.

http://grammar.ccc.commnet.edu/grammar/corresp/corresp.htm
This might help, too.

Once you have written your letter, be sure to re-post if you'd like someone here to give you feedback on your work.

=)