Get a round-sided pen-one that will roll easily. Remove the lid if it has one. You can use a straw in place of a pen, if you like, because it's lighter and moves more easily.
Place the pen horizontally on a table or flat surface in front of you. Make sure the pen has at least several inches of room to roll away from you.
Put your hand horizontally with your thumb facing toward you several inches above the inner edge of the pen as if you're about to cast a shadow over it. You'll move your hand over the pen like you're casting a shadow on it as you secretly blow on it to move it.
Blow on the inner edge of the pen using your nose if possible. At the same time, move your hand away from you over the pen at the same speed the pen is moving.
Use your mouth to blow if you can't blow strongly enough with your nose. Do it discreetly, as if you're simply exhaling instead of blowing. This step may take lots of practice and you may need to put a mirror in front of you to observe yourself.
Stop the shadow over the pen when the pen stops moving. Move the pen back to the starting position if you'd like to perform the magic trick again.