go up to them in confidence when the times right..not when they are busy.
-confindence is HUGE(posture, smile)
-plan what your gonna talk in advance (like how was that test? or work, etc. idk your situation) start with the most banal things first.
-eye contact (dont look away, but dont stare.) and make sure u acknowledge by agreeing/disagreeing(oh ic, thats cool, really?? i didnt know...etc) with them in a nice way.
-laugh, take it easy
- and dont forget to say bye.