I thought you were looking to flash the Alliance ROM? If that's so and it's a JB ROM, then do this. Copy the Alliance ROM file (or any ROM file ... just flashed SuperNexus 4.2.2 to my S2 this morning) to the root of your /sdcard partition or put it on an SD card. Don't forget gapps if that ROM requires it, since I'm not familiar with it.
Then flash the Siyah kernel to the phone through Odin. If you copied the ROM to an external SD card, put it in the phone, otherwise just boot into recovery. Because you are coming from GB you should do this ... wipe cach, wipe dalvik cache, factory reset/wipe data. Now still in recovery flash your rom and then gapps. Reboot the phone and you should be good.