First you'd assign a static IP to your NAS. This would be on a different subnet than your home network - for instance if your home network uses 192.168.1.x addresses, you could use a 10.0.0.2 or a 192.168.0.2 for your NAS. Set the subnet to 255.255.255.0. Set the router address to x.x.x.1 (e.g., 10.0.0.1 or 192.168.0.1 in my two examples).
Then you'd assign a x.x.x.1 IP address above to the PC ethernet port that you plan to use (as a static IP address), and connect a gigabit ethernet cable directly between the NAS and the PC.
You'll be able to access the NAS by entering the NAS IP address into the file explorer address bar (e.g., \\10.0.0.2), and you can also access the NAS's web admin interface in your browser with https://nas-ip-address/admin (e.g., https://10.0.0.2/admin).
FWIW, I don't recommend doing this unless you have a very strong privacy requirement and also have reasonable networking skills. It generally works out better if you just connect the NAS to your network, and limit access to the shares using the network access settings on the share settings wheel. If the NAS is connected to your home network, it will automatically sync time, download hot fixes and antivirus updates, and notify you when there are firmware updates.
