It'd depend a little on just how familiar you are with working on your PC. Objectively, it's not particularly difficult, with maybe a couple considerations based on hardware specifics.
First thing to look at is what your computer/case can physically accommodate. For instance, for a laptop, you'd (likely) be talking about swapping an SSD into the only available space for a drive, meaning your HDD would need an external enclosure, or maybe an adapter to put it in place of your optical drive. In a desktop, you can almost certainly just install both drives, but you may want a 3.5" adapter to make mounting it easier.
Second is what the capacities of the drives are, or maybe how extensive you want the refresh to be. If the SSD is as large or larger than your existing drive, then the simple thing would be to clone the hard drive to it. An SSD may include software to do that, or there are
free options. I'd say to just disconnect the HDD at that point if you're not physically swapping things around. Establish that the SSD is working as expected as your boot drive, then wipe the HDD and reconnect it or use as a USB drive.
The longer process would just be to take the HDD out, install the SSD, and set up Windows fresh. Hook up the HDD and copy things over manually as desired. That'd take longer, but might be preferable if you want to upgrade or just could stand to purge a lot of older data. It also might be the simple way to do it if your existing drive is particularly large and full.