This refers to the length of the VOWEL in the stressed word or syllable. Every word in American English has a certain syllable that is stressed. In one syllable words, obviously, the whole word is stressed. The vowel holding together that syllable is LENGTHENED. Instead of maybe holding out the vowel for a second or half a second, one holds it out for 2 seconds or so (depending on how much emphasis one wants to add).
This part is easy. One says the stressed syllable or word LOUDER than the unstressed syllable or word(s). I have not met any client yet for whom this part of adding stress was difficult. It comes naturally.
This refers to the pitch one uses for the stressed syllable or word. Pitch is the frequency (or the ‘high’ or ‘low’ sound or tone of the voice. It actually has to do with how fast or slow the vocal folds vibrate in your throat, but you don’t need to remember that. Just think of singing. You can make a high tone and then swoop it down to a low tone.
In American English, to add emphasis or stress to a syllable or word, we go UP. We bring our tone up on that one syllable. We do this as we are also speaking LOUDER and holding out the vowel LONGER.
Changing our speech in these three ways (LONGER – LOUDER – HIGHER) on the syllables or words we are stressing imparts meaning to our words. The person listening knows that the stressed words are important, that there is heightened emotion behind the words. That you MEAN it!!