Yeah. Just think of all the instruments being in the key of C. Then if you have something like a Bb Clarinet. It would be written a major second lower, because Bb is a major second lower (2 steps) than C. Follow the same intervalic transposition so if the guitar part was in G major. The Bb Clarinet would be in F major. Now if your writting a song in G major, and you want to figure out the horn part on your guitar. You would do so in the key of G major, then transpose it to F major for the Clarinet player. That's the only thing I may have left out. You probably knew that, but just in case.