Well if you have fretless bass you should do it, it's easy all you need to know is where each tone is placed for example with standard GDAE tuning over the 7th fret on G string you'll get D tone ... (check for other tones on the net) and remember that you must touch the string directly over the fret. Ok now here is how you do it, touch the 7th fret (any string) and when you get harmonic just press it down gently and pull it down the fretboard and that should do it (it sould take only a few minutes of practise just to get the feeling for string).