You flash the Modaco kernel via Odin, if your file system isn't ext4 (which it won't be then you may need to manually reboot your phone - My phone still rebooted automatically coming from a rfs file system)
Once your phone reboots it should enter recovery automatically and you just sit back and let CWM backup / covert your file system to ext4 / restore your rom back on to your coverted file system
(I believe there are instance also when you phone does not automatically enter recovery on reboot, in this situation you will need to manually enter recover after flashing the kernel in Odin - to do this you hold down the power button and volume up)
Another thing worth noting, i had encountered an issue during the conversion process where CWM was not able to restore my existing rom, when this happens you can download a ext4 rom (i.e. modaco rom or overcome 1.1.3) and flash the file via CWM - i.e. you can leave the kernel and try another Rom.
Hope this helps.