Expansion cards are plugged into expansion slots. There are several types of slots:
There is no need to be afraid of inserting the wrong card in the wrong slot as this is impossible because each card type has its own slot size.
To insert an expansion card, just remove the corresponding blank plate on the PC case, then slide the rear of the card into the slot, gently pushing in the front end, then finally screw it into place.
Wherever possible it is advisable to leave an empty slot between cards to allow for improved air flow.