PLEASE HELP ME TO WRITE A LETTER

helo dear, im just a new member here on this site.first of all im not really confident with my grammar. please guide me to write a letter regarding with my numerous errors i made.i have received a memo from our hr regarding with my numerous reversal with my cash transaction which was due to mislooked on such details and sometimes reasons like double posting due to offline processing.i was asked to write a personal comments on it.please do help me how to begin it.

thanks