OK, I'll take the OP seriously for a moment...
'How' you say "No" must surely depend heavily on the circumstances and context in which one is called upon to respond in the negative.
As an added complimication, it is occasionally acceptable to say "No" when one actually means "Yes".
For example, when a leddee of the opposite sex asks, "Does my bum look big in this?", the correct reply is always "No". But never try to justify that 'no'. To do so opens a ch1nk in one's armour that is sure to invite a stringent piercing akin to a misplaced 'Prince Albert' to one's jap's-eye.
If in doubt kids, remember, the drugs don't work - - - just say 'no'.