mmm....I could be really condescending here and say it's all about first, third and little finger left hand technique....(how many of us play ghost notes with the second finger of the left hand - assuming you are right handed of course) - however, it is horses for courses and experimentation is good... for instance, I actually play ghost notes with my right hand with the little, fourth and third finger tapping on the fretboard in sequence to create a triplet, add in the thumb (right hand) and third finger again (left hand) I get five 'taps'..f**k I must have been bored that night....
Bottom line is don't be afraid to just try stuff....heck, we haven't even talked about plectrum and finger style combined to play ghost notes, include the thumb and we can start talking about James Jamerson and Brian Wilson.....sh+t, I'm even boring myself now.....