As I wrote in Arrangement, Orchestration or Transcription, when scoring for brass I prefer to work from keyboard music. Even when doing a transcription, I'll look for the composer's piano version if available from which to work for a couple of reasons. First, it's easier - everything is in C (ever had to transpose horn in G or a D flat piccolo part). Second, I'm less likely to be influenced by the composer's choice of instrument.
Perhaps most importantly, it provides some insight as to how pianistic effects are translated in the composer's mind to orchestral effect which, in turn, suggests solutions to otherwise thorny orchestration problems. A left-hand octave tremolo, for example, might be orchestrated as a whole note for double bass and cello with a timpani roll to provide some energy to the sustained sound. Often that which seems like too many notes in a piano score is simply the composer's desire to compensate for the piano's natural tendency for sound decay.
So I'll grab a piano score and get started. A two-piano version (or one piano four hands) is even better.
The first task is to get all the notes into my score. I'll null out all my instrument transpositions and enter all the notes in an essentially SATB (or variations thereof) format. I'll disregard instrument ranges - in fact, I want all the original octave registrations correct and intact. If there is a passage that is homophonic (like a 3 or 4 pitch repetitive chordal accompaniment) I'll enter those on a single line and then let Finale's powerful 'explode' function sort it out later. I'll not worry about things which are seemingly too difficult for a particular instrument - just get the notes into the score. This is purely note entry time.
Save your work. Play it back through MIDI and listen for bad notes. Make corrections. Save your work.
Next up - it's All About That Bass.