If Windows is not yet installed, the procedure is this:
Start the Windows installation, choose manual installation, create a partition for Windows and the rest you don't have to touch, this will be the raw partition that AROS will recognize as its HD, so summarizing. no initialization (MBR) and no formatting.
If Windows is already installed, if you have a second partition "you have to delete it" and leave it "clean" without performing any operation.
If you don't have a second partition, you have to "shrink/divide" the Windows partition and leave an empty partition without touching it, that will be the disk recognized by AROS.
Before shrinking Windows, do a defragmentation to sopst all the data in the initial part of the disk, if you have a Solid State HD this operation is not necessary.