So once you've installed the SSD.....boot to your Windows install media and it will run Windows setup. During which you can select your new SDD drive to install the OS to.
You can create a bootable Windows install media buy using the Windows
Media Creation Tool
You can leave the current Windows install on the other drive for the time being.
With Windows installed on both drives, when you boot up your system, you will be prompted to which one you wish to boot to...you can change the default by following
this guide.
If you are happy with your new Windows install, you could either reformat your SSHD fresh, or simply delete the Windows directory and program files from it to free up space....or you could just keep it as a backup OS drive.