I light my farts all the time, usually when I'm home alone because I no longer have any friends.
The key is not to do it bare assed, but not wearing jeans or anything either. Ideally, you just want to be wearing underwear (tighty whiteys if you want the full hilarious effect).
Don't lie on your back, sit on the very top of your butt and lift your legs as high as you can, this will concentrate your fart stream.
Position the lighter 2 inches from your asshole (1 is too close, 3 is too far for maximum effect).
Let her rip. The good, loud farts produce the most fire. SBD's are more potent, but usually not enough gas to produce a big flame. Beans, kimchi, and cabbage are all good foods to eat to prep your stomach for a light show, but honestly I get better results from drinking lots of milk beforehand.