If I were to add additional RAM, is it as simple as just turning off the PC and putting the cards into the open slots on the motherboard or does anything else need to be done?
Yes however check which slots are best to put ram in (it's different for each motherboard) e.g.:
0 1 0 1
sticks in the 1 slots.
You need to get the exact same ram for the best chance of the sticks being compatible.
If everything is compatible, the computer will automatically use it. You don't need to do anything else.