Take the trombone part and take it up a perfect fifth. C becomes G etc.
However there is a little trick, read the trombone part in bass clef as
though it were treble clef and then play one step lower (like you were
transposing Eb horn music), so that second space C in the bass clef would be
A in treble clef, simply take it one step lower and you would play a G which
would be the correct note to play. The thing that you must watch out for,
however are key signatures, so if you are playing trombone parts in Eb
major, then the F horn parts would be in Bb major. When I do this
transposition, I usually pencil in the key signature to help me remember.
I hope that this helps.
Mark L.