How do you transpose from trumpet to French horn?

You play a whole step down from the written note. When an F horn player is asked to play a written C, the note that will come out will be an F concert pitch. When an Eb horn player is asked to play a written C, the note that will come out will be an Eb concert pitch. That is why they are called F and Eb instruments. So when an F horn player is asked to play a C written for an Eb horn, in order to play the concert Eb, he will have to play a Bb. You will be adding two flats to your key signature (mentally), and if sharps are in the Eb part's key signature, you will cancel two of them. In the case of the key of G major, where there is only one sharp, you will cancel the sharp and add one flat.