I agree. If the Fusion drive works... Fishrrman's way is fine.
But to your question, OWC and iFixIt have directions for these.
If you're going down that road, first hook the new SSF up using a USB 3 dock like this.
https://www.amazon.com/Sabrent-External-Docking-Station-DS-UBLK/dp/B00IKAQ538
Next install the desired OS onto it.
Boot or Option-boot into the external to make sure it fires up but don't do anything else. Shut down and disconnect the external.
I'll assume that you've already pulled your info off the 2012, right? If so, wipe this Mac clean—this will destroy the data on the blade part of the Fusion drive—you'l deal with the HHD later. If not, see *
Now install the new SSD into the 2012.
You can make a new Fusion drive between the blade and SSD (Google is your friend). I don't recommend it but you can if you keep the blade in there. You can get a few $ for the used blade, however.
Your dad has an older Mac? If so, *connect them via ethernet. Use Migration Assistant from the 2012 to pull everything over.
If he doesn't then set up the new (to him) iMac as you would any other.
Now, using that external dock, set the old HHD into it and wipe it writing to zeros 7 times. This was not necessary with the blade—TRIM made the data unreadable within a minute after you wiped it.
Sell the blade and recycle the HHD.