Change the tuning of your guitar from the standard, EADGBE, tuning to open G, DGDGBD, or any other open tuning. Open tuning is used with the slide because a chord can be voiced by placing the slide along any one fret. With open tuning there is no need for difficult finger positions when playing a chord.
Place the slide on the pinkie finger of the hand you use to hold the neck of the guitar. For right handed players this will be the left hand. The tip of your pinkie should be exposed so you can feel the strings that you are playing.
Hold the slide perpendicular to the strings and only press lightly. Pressing too hard on the frets will create an unwanted buzzing sound.
Cover only the strings which you want to sound when playing. By not covering the other strings with the slide you will not have to dampen the sound from the unwanted strings.