This has been explained in many threads already, but you have already asked so lets go.
Before we start, do you have Gingerbread bootloaders installed? If you don't, your recovery will be rainbow distorted once on ics or jb. Some people can live with it, some not.
You need blue recovery to flash JB. You can get this in two ways:
a) flash any ics rom following the double flash instructions since you are coming from GB.
b) use the ics one-click recovery linked in cm10 OP
Once on blue recovery, follow the directions of whatever JB rom you want to flash. These will indicate you to wipe data, cache & dalvik and format /system and the flash the Rom.
Sent from my sweet & buttery Infuse