Listen to the metronome beat. Set it to a tempo of about 72 beats per minute or base it on the tempo of the music you are trying to play.
Tap your foot to the beat to get a feel for the pulse. Begin by playing three equally spaced notes per beat. This is going to be your subdivision for counting quarter note triplets. Quarter note triplets can be subdivided with eighth note triplets since two eighth note triplets equals one quarter note triplet. Therefore, six eighth note triplets over the course of two beats equals three quarter note triplets.
Count quarter note triplets by internalizing the three notes per beat you played in the previous step. Once you feel confident that you have internalized the beat instead of playing three notes per beat, play every other note so that you have a total of three notes over the length of two beats.