I would type it out so that it is easily read ..
Start by addressing the situation and then go into what lead up to it and how you now feel after your actions .Also point out tge impact your actions have already had on your life , health and future . Take full responsibility and then show how it is completely out of character. Then outline what you do in your work , home and family life and the shame it’s made you feel .
I did 3 A4 pages as mine . My opinion is that you can’t write enough when you are defending yourself and also more importantly apologising .
Be humble and honest and remorseful.
Good night