It's better to put it between your balls and thighs. Then use your hand to slightly rub it in and up your crack.
You have to make sure you thoroughly clean yourself though and be careful which brand of deodorant you use. Sure and degree works best imo. There's a few that'll burn like hell... I remember arm and hammer feeling like fire.
Idk if there's any side effects, but I advise against getting any in your butt or pee hole for whatever reason...
I'm sure you'd get cancer or ibs or some shyt. You'd smell pleasant whilst dying slow tho.