>4GB on Amiga 1200 / 3.1 Workbench?

[adolescent]

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.

[adolescent]

IDEMax is just the new/current name for IDEFix.  They are the same product, and are both commercial.