There are two basic kinds of throwers, injected kinds and ones that rely on unspent fuel, the later is the safest and most common,
You set up two things.
1) an interupt switch to temperarily disrupt the sparg plugs from firing, you blip the throttle flip the kill switch and fuel gets sucked out through the exhaust
2) a spark plug at or near the tip of the exhaust to ignite the flame
This kind will kill any cat you have installed and will work with FI or carb cars

Number 2
insta spark plug and a fuel injector in the exauhst or an extra set of tubes out the back and let her rip.


Lots of kita are available, here is the awesome kit that will find its way into my ride when its done.

http://www.autoloc.com/detail.lasso?itemid=flame2

This kit is adjustable, from 2 foot long blue flames to 20 foot long yellow barn burners. There was a whicked writeup on them in the March 2004 issues of Street Rod Builder Magazine.