Step 1: Getting ready
tell them that it took you a loooooooong time to master the ancient art of levitation, then say that you must concentrate very hard and fully believe that you can defy the laws of gravity (that is the "hardest" part of it), then say that you do a slow natural "jump" and stretch your entire body up to lift you body off the ground. Since most people can not levitate tell them that it is almost impossible to levitate any higher then a few inches. it is "very hard" to ace a landing which means that even if your only a few millimeters off then you will stumble and most likely tackle the nearest person or break a priceless vase(if you have any). And that is what you MUST tell them or else I will personally smack you on the back of the head.
YOU HAVE TO TELL THEM THIS(all of the above) AFTER YOU HAVE "LEVITATED"
but befor you levitate do not tell them what you are about to attempt does not work all the time but you must tell them to back up (or else they will see what you are doing.