Um just tell them? I generally pull my friends over at recess and just say the words and be done with it. I drug one of my friends into a dark empty locker room and came out it was really funny xD
*Badum tsss* If you're in a group setting, you could do what I did an ask everyone "Are you going to hell? Why or why not?" And once it gets to be you say "Yes, if being gay is wrong." and let it sink in. If it's just one person, try to get a conversation going and just try to work it in as a joke or pun. I really don't have any off the top of my head.