Reddit mentions of Complete Handbook Of Voice Training

How old is the daughter?

If she's under the age of 13, I'd suggest doing nothing more than teaching her to match pitch and some music theory on how to read sheet music.

I strongly suggest this book for you to read (it's not very sciency, very easy to understand) if you're trying to do anything else.

If you are not comfortable teaching voice lessons, it's okay if you turn your coworkers down. I don't personally recommend giving voice lessons unless you're secure in what to teach, and if you've been formally taught to a comfortable degree. Being in charge of someones voice is a responsibility, and while you probably won't do anything to permanently damage them, you always could. They put full trust in you, so it's important for you to know what you're doing.

Start with the basics: Posture, breath, and matching pitch. Until the student has a basic grasp on those three things, nothing else is going to stick terribly well. Only mention something about an open throat or resonance or tension if it sounds (or if the student tells you) that it physically hurts.

I'd also recommend getting some formal voice lessons yourself if teaching is going to be a thing you continue to do. Not only will it improve your own personal singing, it will improve your teaching as well.

Good luck!

From what I've read tone has a lot more to do with your breathing and posture than you would think. If you have enough air in your lungs to back up your voice the note will come out with more strength behind it.

Your throat should be open. Go ahead and fake a yawn. Open like that. It should be open enough that when you take a breath you shouldn't hear it. Of course, the guy from Muse gets by fine with gasping.

Your Adam's apple should not go up when you sing a higher note. If you do that you'll end up squashing your voice. I'm horrible at not doing this, but supposedly it's possible.

I've also found recording yourself will help. What you hear while you're singing will never sound like how others hear it. Your voice is vibrating your body so it will sound deeper than it actually is.

I bought a copy of this book. The above were the points I remember from it. Note: I still can't sing :)