I don't have a step by step, but there are a couple of things you need.
1. An updated scsi.device that supports HDs larger than 4GB. (This comes with OS3.5/3.9, IDEFIX or IDEMAX)
2. A filesystem that can support partitions above the 4GB barrier (OS3.5/OS3.9 FFS, SFS, PFS, etc.)
Note: since these patches aren't in ROM, you'll need to keep at least the boot partition in the natively usable area.