the next to the last flat tells you the key. this is a particulary hard key to play in - so if you have to write in a flat next to the C and F in your music - you might remember to play them that way! after a while it becomes easy to remember to flat everything. your mind starts helping you think of the notes as 1/2 step below (and not just black notes, but also white notes - as in the case of C and F - since the white notes are the closest 1/2 step down).