I can't speak to the Kindle Fire, but I did it to my wife's Nook Tablet. It was moderately easy to do. As Grubbster mentioned, XDA is about the best site for how-to's on that sort of thing. I also found that the process got more refined and easy as more people did it. Any glitches and problems were discovered on the way, and then rev'd into the process. Most of the people that end up bricking their devices don't follow the steps properly.
Sure, you could brick it. But you could also drop it by accident and brick it that way also.
"you know the road doesn't end, when it reaches a bend..." - Poco
Proud Supporter of JK Knives #65