
Howard F. answered 05/20/20
ASCAP Songwriter with DMA in Composition and 20+ Years of Teaching
The best approach for transposing instruments, IF you con't feel comfortable writing/reading the transpositions is to write them at concert pitch and use notation software to automate the process of transposition. What matters most is for the player to receive a part that is properly transposed. You can even decide to leave the score entirely in C (concert pitch), as long as you indicate that clearly on the score, should you be handing such a score over to a conductor.
Of course, there's no substitute for practice in transposing, both writing and reading such scores.
A good set of principles/practices are as follows:
For most Bb transposing instruments, just think of them as sounding a whole step (Maj. 2nd) below notated pitch or notate them a whole step higher than you want them to sound.
For A transposing instruments, think of them sounding a Min. 3rd below notated pitch or notate them a Min. 3rd higher (one line/space on the staff higher) than you'd want to them to sound.
Horn in F is a little bit of a strange animal in the sense that in older scores, you will sometimes see them notated in bass clef a 4th lower than sounding (instead of a 5th higher than sounding, which is the modern norm). Also Horn in F is usually written without key signatures and instead just uses accidentals.
There are also a few instruments that transpose up rather than down, so you just need to be aware of those. A good book on orchestration, like the Samuel Adler, Kent Kennan-Donald Grantham or Walter Piston books can provide detailed info on transpositions.
Again, the more you practice writing transpositions, the easier it will become.
Good luck, and feel free to ask me any questions by message.
-HowardF