1. Always start with good posture
One of the common mistakes I've seen is blowing softly but with no breath support. Demanding good posture from the beginning has cut down on most of those for me! I use a simple hand sign cue to have students sit up in their chairs any time they sing or play a wind instrument, so I use that even to have them all sitting up even when I first introduce them to how to blow on the instrument (read more about my hand signals in this post).
2. Say, then whisper, the number "TWO"
Once they are all sitting up correctly, I have them first say the number "two" out loud. Then I tell them to whisper it. Easy!
3. Hold out the whispered "two" longer, then softer
Now it's time to turn the number two into supported, controlled, and tongued breathing. I tell them to whisper the number "two" again but hold it out longer (I usually just do 4 counts- I don't want them to get in the habit of breathing until they run out of air). Then I tell them to do it again but gently. I talk to them about how the recorder is a beautiful instrument, not an obnoxious one, so if you blow too harshly it won't let you make a nice sound. The kids seem to respond better to the word "gentle" than "soft"- somehow more of them continue to use a supported breath when I say "gentle" than when I say "soft". I actually go around the room and hold my hand in front of their face to make sure they are blowing softly, and I have them check themselves with their own hand as well.
4. Blow into the recorder with a low G fingering
The first time I have them blow into the instrument, I start them on a G. It's easy to teach the fingering, and it's easy to get the correct note without being too easy- they still have to control their breath to get the lower note. At this point I also make sure they aren't taking too much or too little of the mouthpiece into their mouth, and also check that nobody is biting the mouthpiece.
And that's that! How do you teach students to blow into their recorders correctly? Share your ideas in the comments!