We built our first FlashPool aggregates just a few months ago (i.e. be wary of any advice I might have) but we did research it a bunch beforehand. Anyway, we ended up creating storage pools so that we can easily add SSD capacity on down the road. Our build ended up being:
Storage Pool: 11X3.8TB SSD (1 spare) - 38.4TB total
Aggr1: 42X6TB SATA + 19.2TB SSD (50% of the storage pool)
Aggr2: 42X6TB SATA + 19.2TB SSD (50% of the storage pool)
Essentially, provisioning a storage pool allows you to add "capacity" to the SSDs (in the form of new disks inserted into the shelf) and then distribute that capacity to the spinning disks. We gathered that the common logic is that if you're not going to be changing the SSD capacity, you can just add the SSDs to the spinning disk aggregate and call it good.
So the steps are basically to build the SATA aggregate(s) as you see fit and then build the storage pool (if you want to go down that path). Once both sets are provisioned you can add the hybrid capacity (or SSDs) to the spinning disk.
Quick storage pool write-up: https://library.netapp.com/ecmdocs/ECMP1636022/html/GUID-6F50251B-8D13-40FA-A17B-510320CF2E3C.html
Hope that helps,