You would just attach the portable SSD to the computer and when you boot the Windows (OS) disk chose the SSD as the drive you'd like to install to. To be safe I'd remove the internal HDD first so you don't inadvertently install to the internal.
Another way could be just to clone your internal HDD now with something like AOMEI Backupper to the SSD and then you'd have a 1:1 copy of what you're running now without having to install a fresh copy of Windows.
In either case you will have to go to the BIOS/UEFI boot options to boot from the SSD each time you want to use it.
Have you ever installed an operating system before?
Another method which would be a lot easier, but depends on what that one program is that you want to run is just using a virtual machine. This is software that runs in Windows and allows you to boot an operating system from withen Windows in its own environment. That's why it's called a virtual machine. I prefer and recommend the free VMware Workstation Player. It'll allow you to run such operating systems like XP, Vista, Windows 8, Windows 7, Windows 10, Linux and its derivatives. I even think Windows 98 and maybe DOS.
Again, this depends on what it is you're running. You can't run a program like a game in a virtual machine. But you can run all kinds of other stuff. Be it a browser, Office, an email application, numerous programs. I use a few Win 7 VMware installs for testing things and to isolate programs on how they work in one OS versus my current OS install with that program.
In VMware you just go to the install option and point to your optical drive that has the Windows disk. That's it. It'll install. Upon first installation you can enter the product key or you can do it latter and just have a 30 day install. If I can remember right, Windows gives you 30 days until you have to enter the product key.
Since this OS is all software-based. I took my VMware install, compressed it with 7-Zip and burned it to a 50 GB Blu-ray disk. LOL! You can even use a tool from VMware to create an image of your computer and run that in VMware.
You'll want version 15.*. 16 Won't run in Win 7 apparently.
VMware Maintenance
Free Backup Software for Windows 10/8/7 - AOMEI Backupper Standard
