Install the drive, boot up in recovery mode. It'll see the new drive + SSD, auto-run diskutility and make one for you if I remember right. I just had to click 'ok' to confirm that is what I wanted to do. It might have made me delete the 128GB first since it saw the 'broken' fusion drive. I can't remember, but it was extremely easy. I just did it a month ago. 6.1TB of storage. I used the kits from OWC.