This human interest in speechifying has always mystified me. The one thing that puts me readily to sleep is to have someone climb up on a stage and start off with, 'Ladies and Gentlemen...' OR 'Respected/Honorable so-and-so...' or some such variant that indicates that he is teeing off to unleash a flood of words on you. And yet, there are oodles of consultants and clubs and whatnot that have made a good living off teaching people how to talk the ears off a captive audience.
Given this, is it a surprise that Tiru has spent a few of his kurals on teaching people how to speak in public? I mean, no matter what he personally thought of this exercise, his readers would have refused to accept him as an expert unless he proffered advice on this facet of human relations. To become an expert in the eyes of Society you HAVE to be an expert on what Society wants you to be an expert in. Else you will be ignored. If Society wants you to be an expert tie-tier (I mean tying that abomination that strangles your neck) and you refuse to do that, you'll be consigned to the role of village idiot, no matter how much of an expert you are in everything else. THAT Tiru well knew and so...
Vagaiyarindhu vallavai vaaisoraar sollin thogai arindha thooimaiyavar - Tirukkural
They who know the way of words AND are capable of assessing their audience never falter or fail in their speech - Loose Translation
You need to understand that you are assumed to know the subject matter upon which you intend to speak. THAT is a given, the basic ability that is assumed. Thereafter, you need also to know HOW to speak.
The problem about how to speak is that it is not the same thing everywhere. If you are talking to school children, say, trying to explain the theory of relativity, you'd need to speak of it differently. You cannot talk about it the way you talk to physicists OR even the way Einstein simplified it - 'Sit on a hot griddle vs sit by a pretty girl'. I mean, depending on the age AND assuming that children of a certain age were like us when we were children (The 'Girls! Ewww' OR 'Boys! Ewww' age!) your audience may not see much difference between the hot griddle and a pretty girl.
There is the maturity level of the audience, there is the background of the audience (I mean, using, say, carpentry examples in a group of Insurance adjusters or the vice versa would not work well, would it?), there is the bias of the audience (The way you say things that they want to hear versus saying those things where you want to convince them about) and so on which needs to be accounted for to speak the right way to the audience.
So, when you talk about 'How to Speak' you talk about knowing a wide variety of ways in which you approach a subject. And THEN you need to understand the audience you are addressing, pick THAT method of speaking which best suits that audience. Then, you can speak both with confidence AND without giving offense.
As usual, Tiru's advice is easier said than done. But then, what worthwhile advice isn't?