If you would like to propose to a girl with a letter, here is a simple template you can use to write a letter asking for her hand in marriage.
I will get straight to the point. I love you more than anything. I am not ashamed to admit that to anybody. When I feel down, I immediately feel great when I look at you.
There is no better time than to ask you this important question: Will you marry me? I can’t imagine spending the rest of my life with anybody else. You define what life is to me. Nothing makes me happier than being beside you. Hence, I will be extremely satisfied when you walk down the aisle with me.