Legalised Translation of the Letter into English:
MUSIC SHEET
Look for a church which has words of this song written on its front facade. Count all altars inside and set off down the street named after a writer. Divide the number which you found into two even ones, and between houses with these numbers you will find a treasure. You will get to it easily when you gather some information about the church in a correct order: number of steps of the nave's rooftop, number of crosses which the church patron saint sees on the pavement below, number of stairs and ladders leading to the upmost cross, and height of this cross in metres.
PHOTOGRAPH
The church was built here.
Good luck, yours loving M.