1) Any boy over the age of 12 who knows what "masturbation" means, is already doing it! ROUTINELY!
2) There is no need for anyone to be "taught" how to masturbate. Most kids figure it out on their own. It's one of those little biological facts of life. And if they're dumb enough NOT to be able to figure it out for themselves, then they don't deserve to reap the benefits of it.
3) It's always nice when parents can be open and honest with their children, but some lines need to be drawn (if only to save the embarrassment). Any kid that is so desperate to know "how" to masturbate is better off seeking advice from a male friend his own age, not "dad".
4) I'm pretty sure that the prudish, sexually infantile society we live in would consider any such "instruction" to be an illegal, incestrous form of "child abuse", so from a legal standpoint you probably shouldn't be exposing yourself to them sexually or having them expose themselves in front of you sexually.
5) I am all for the "endorsement" of masturbation (at any age). In fact, if you don't want your kid to be sexually active at a young age, you should only HOPE they are masturbating and only masturbating until they are emotionally prepaired for the consequences of having sex.
6) This is how my masturbation instruction would go: