How to play the Lap Harp
Hold the harp on your lap, with the base on your thighs, then rest the harp against your right shoulder. Your thumb and first three fingers of each hand should be on the strings. Your little finger isn't used to play the harp.
Your right hand plays the upper octaves and your left hand the lower. Keep your fingers slightly curved and your thumb high. As you play, your fingers will snap into the palm. Only play with the edge of your finger or thumb. If you use too great a surface area to pluck the string, the sound will be unpleasant.
Practice until it sounds right. You might find that a harp teacher or teach yourself book is a shortcut to good technique.
The video below is an excellent introduction to playing a lap harp.