Well, 2 ways to achieve it. the first way is deliberate, tapping the fuel lines into the exhaust and putting an igniter near the exhaust tip

The other way (or rather not a way but a reason for the flame) is a BOV. when the car boosts, the MAP sensor detects X amount of air going into the engine. When the BOV does its job of releasing air (Y amount), the amount of air is now X-Y cos the map is located before the BOV.

Now there is too much fuel for too little air. The excess goes into the exhaust and ignites when there is sufficient air, usually after the muffler.

 

If you're backfiring a lot, does that mean you're running too rich?

 

if youre on a stock BPV, yes. Youre rear bumper & exhaust tip will also start looking brown/black.
Like this:

or this:

As stated, getting a vented BOV & CAI should do the trick.

 

I doubt you could get a backfire through the stock exhausts -- 3 mufflers and a cat are gonna eat those up. I think I got them when I disconnected it just post-cat though.