You really need to read the posts in the upgrade subforum before you start flashing or you can wind up with a "brick" (e.g. unusable device) Learn the terminology and the whole kitchen/cooking process. You will get a good idea of the problems that you can run into. If you really get into a bind on something that noone has seen before, there is plenty of help. But you should try and find it first on the forum cuz 9 times out of 10 someone else has worked through it.
Here's a good tutorial on the process here by Ga.Medic and others:
http://forum.ppcgeeks.com/showthread...light=gguruusa
Good Luck and have fun but be careful when you just start out and make sure you have some idea of what is going on.
I really don't know about the Cricket carrier as I flash with Verizon specifics. This will be more challenging if you purchased a XV6700 (for Verizon) and you want to change it to Cricket. Unfortunately, I don't know about those carrier specific parameters.