Reddit reviews R&B Horn Section: Transcribed Horns

The main tip is know your chords and know how to go from one chord to another with guide tones. You could definitely transcribe horn riffs from Stax etc (Chicago, too!) recording, and there's a series from Hal Leonard that has the horn sections already scored. Horn stuff is also good for phrasing, as those guys have to take a breath to keep playing. ;)

Another thing you could do is cop lines from harmony vocals.