That's a slimline case, which severely limits your upgrade options. You can add a card, but you'd have to look for low-profile cards since full size cards won't fit in that case. Low-profile cards are limited to low-end video cards since more powerful cards need a bigger circuit board and bigger coolers.
If you do add a card though, you're basically right: shut down the computer and unplug it, remove the cover from the slot in the back, plug the card into the long slot (a low-profile card won't need an extra power plug), restart and install drivers.
Thank you very much, that's exactly what I was looking for
I'll go check some stores for a low-profile card right now then. I appreciate the response!