Study how to transpose notes up a perfect fifth; the chromatic scale aides in transposing notes. To transpose a perfect fifth, you will need to count seven half steps. With this system, a perfect fifth above Bb is F. Notice that F is both seven half steps and 5 note names away. E# is enharmonically correct and produces the same sound, but it is a different note name.
Transpose each note from the trombone up a perfect fifth. Because the French horn reads treble and bass clef, you can use either clef. Choose the one that will have the least amount of ledger lines.
Decide whether the transposed notes are too low for French horn. This will only be a problem if you are using pedal tones on the trombone. The French horn can play down to a Bb two octaves below middle C.