There are a few different ways you can achieve your goal here. Basically you'll need to download if you haven't already the OS installer from the App Store. Use the purchased listing within the App store app to locate the download as the store front no longer offers the older OS's.
Once you have the OS installer you'll need to think though how you want to do this next step. I would recommend first creating a USB thumb drive as a bootable drive (16GB or larger) using a copy first of the OS installer so you can install the OS onto it. Or, you can leverage your current HD by using an external drive case to hold it. That way you can use the OS installer from either.
OK, now the next part here getting the needed hardware and installing it. You'll need to get the 2nd drive kit to support the 2.5" SSD and you'll need to get your new HD & SSD.
I would recommend getting a 1TB drive Vs 750 MB you have listed it would be a better investment given the slight price difference. Remember the SSD storage space is not additive to the HD so you will only 'see' the HD's space as available.
Once you have things installed following the IFIXIT guides. Use your old HD or USB thumb drive to boot up your system to format your HD & SSD as GUID and partitioned. You'll only want to install the OS onto the HD and if you want you can also migrate your apps & data back as well. Then using the command line setup the Fusion Drive set with the SSD.
Take your time and don't force things as the connectors and space things fit into is very tight and easy to damage.