How to get myself up and running

You missed a few steps and just down the wrong path ;-}

Your new drive needs a special cable as its not an 'Apple custom drive'. Here's the cable you'll need: OWC In-line Digital Thermal Sensor for Hard Drive Upgrade for 27" iMacs 2012 and Later without it your system struggle to keep its fan under control as the sensor the system needs is not accessible on your replacement drive. Once you've got in installed your hardware issues should be resolved.

Next, is getting the virgin drive setup using Disk Utility but we've put the cart in front of the horse here as you need to first boot up your system to gain access to this utility. We'll need to have access to the internet so you can boot your system up using Apples macOS Recovery services. The older version allowed you to select from the recovery partition on your drive (in this case not present yet) and Internet recovery. The newer one automatically choices depending on what it found.

Next, we need to prep the HDD with a single partition Disk Utility for macOS Sierra: Partition a physical disk using Disk Utility and format the partition. Once this is done we can move forward on installing the MacOS make sure you install the same one you are currently using. This will in turn will finish getting the drive ready for the next step.

Up until this point we've focus on the HDD but we still have the SSD to rejoin with the HDD to re-establish the Fusion Drive set here's that step: How to fix a split Fusion Drive.

After that is done I would recommend you next go to the Apps store and make sure your system has all the needed updates for that release before restoring your TimeMachine backup.