It’s very simple. To just plug in all you need is a SATA cable and you’d already be able to tell where they go from just looking at the motherboard. The only difficult thing is finding a place for them and putting them there
All jokes aside, installing a hard-drive is SUPER simple. If it's a standard 3.5" spinning drive then you'll need to give it power and data. You connect a PCI cable (which you'll probably find you already have hanging around in your case somewhere and looks like this
Then you'll need to connect a SATA connection which again you may already have inside the case somewhere, this connects the data.
And.... That's it! Just make sure you've secured the drive into the case properly (Most cases come with plastic "sleds" that you screw the hard-drive into. My only other comment would be to ensure you're looking at the direction that you need to make the connections in, as with USB connections, they'll only go in one way!