Simply replace the unfilled areas such as "" and "[date]" with your information.
[Ed's note: maybe consider using this as a speech instead?]
Dear [name of love interest]
You know that I love you, but my love has grown during the time we have spent together. It is not the fleeting love of childhood romance. It is not the fast burning love of a short-time fling. My love for you is solid in foundation, strong of design, and beautiful in construction. If I were able to view my love for you as one views a perfect building, the pureness and beauty of it would bring me to my knees.
And so, I am on my knees, asking of you the only thing that seems important:
Will you marry me?