well u can always hit on their girls or take their attention away and give it to someone else but personally i think its stupid and its pretty low that u have to make a girl jelous, that shouldnt be part of ur game.
jelously is an immature act, making someone else is like pre school,
you should develop a game that u can build attraction and interest WITHOUT beating around the bush, and making some girl jelous is like that..
if u get caught doing it. and if the girl can clearly see that u tryin hard to make her jelous it makes u look weak. very weak.
why would u need to make a girl jelous? if she ur ex? if so she should get jelous by you walking away and moving on that kills them the most