Ive just gotten a digital recorder and I'm looking to add horn parts to my songs. I'm just learning trumpet and I already play tenor and bari sax . I'm just a little confused how to write for the horn section because I know that all three instruments are in different keys. Help would be appreciated.

Trumpets play 1 whole step down
Alto Saxes play a maj6th down
Tenor Saxes play a Maj 9th Down
Bari Saxes play a major 13th down
Trombones play at concert pitch

Therefore:
Write your horn parts out in concert pitch first (as if you were playing them on piano)
Trumpets - transpose 1 whole step up
Trombones - no transposing required
Alto sax - transpose a major 6th up
Tenor Sax - transpose a whole step up (but remember that they sound an octave lower than written)
Bari sax - transpose up a major 6th (and it sounds 1 octave lower that written)

Keep in mind instrument ranges.

Get "Modern Jazz Voicings" by Ken Pullig and Ted Pease. The first half of the book gets you up and running with a very detailed and precise methodology for writing for horns. The second half gets you into the modern jazz stuff, but if you're just looking for more pop/rock/etc stuff, the first half is golden. I recommend that book to everyone, it encapsulates a great deal of the arranging program taught here at Berklee.

Get "Modern Jazz Voicings" by Ken Pullig and Ted Pease. The first half of the book gets you up and running with a very detailed and precise methodology for writing for horns. The second half gets you into the modern jazz stuff, but if you're just looking for more pop/rock/etc stuff, the first half is golden. I recommend that book to everyone, it encapsulates a great deal of the arranging program taught here at Berklee.

Dude, shhhhhh.

That would involve actual "Music Theory". We all know here that Theory is modes and the Circle.

Seriously, you should post a link to your "Drop 2" voicing video. That's also a sure-fire easy to please voicing that Supersax used exclusively.

Listen to examples of music in the style you are writing in. Most of the time the harmony in the horn parts isn't complicated. It doesn't have to be four part harmony. Keep it simple: try writing a melody in the trumpet, then write a harmony line below it on the tenor. A line double an octave lower sounds cool too.

http://www.youtube.com/watch?v=9BDKJK2F-zY

This will give you a very basic idea on how to voice horns.

I know most of my theory but im mostly writing my horn parts on piano.