To use a plectrum, the player should hold it in zer main hand (right if ze is right-handed, left if left-handed) and sweep it upwards or downwards across one or more strings.
Some guitarists (Buddy Holly and many thrash players are famous for this) hit the strings only on the downstroke, making their playing sound very precise. Plectrums are also useful in sweep picking, in which the player plays a series of notes very fast� by sweeping the pick down one string at a time.
There is some debate as to whether plectrums should be used on bass guitars. Many people prefer fingerpicking, although others say they can play better and faster with a plectrum. Since there are sizeable canons of excellent bass players with both techniques, it has to be up to the individual player.