Easy way to do them is to just relieve the pressure from the fretting hand on a given power chord so it dampens the string. The only thing to look out for is the tone (as in note) that the dead note is to be - ie you get a different tone at different positions across the neck. The one thing that differentiates this with harmonics (besides the string postion) is the fact that you leave your finger on the string to dampen the note, otherwise you will get the ringing harmonic (which can work very well when interspersed with dead notes).