Play simple music together.
Imitation is the FASTEST way to learn how to play an instrument.
"I play 4 bars, then you echo what I just played." Then repeat the passage together. Start with whole and half notes. Lay down a big sound for your section to copy. When playing together, this will give the others confidence.
A good book for this kind of workout is the Walter Beeler Method For Tuba, Book 1. (Book 2 is also good, but a bit more advanced.) The Beeler is published by Warner Brothers and should cost less than $10.
Words are going to get in your way--too many words can easily tie a beginner up in knots. Just get them making sounds and trying to imitate your sound.