Yea I was going to do the same, just flash a pre-rooted odin flash. Up to there I believe I will be ok, but after I am rooted and unlock the bootloader, flashing a custom rom is where I am a bit lost.
I would have to save the custom rom onto my phones sd card, boot into recovery (by the way do i need a custom recovery?), and flash the rom correct? May I ask which ROM u are running?
Is there such things as a stock rooted JB rom? with all the goodies such as google now?