Difficult to give precise instructions as we don't know what case and motherboard you have.
Some cases you take the top off and that releases the sides, others you remove screws at the back and slide the sides of backwards, one of mine you just pull a catch and the side drops down with the motherboard attached.
Anyway if you can work out how to get in:
First disconnect from the mains (not for your safety, but because the motherboard could get damaged if there is any power to it while you're working on it. The off switch at the front of your machine does not actually switch it off, but just puts it into standby).
At the back of the motherboard you will see some cream coloured slots. They face a panel with removable blanking strips. Some machines the strips are screwed in, others you have to punch them or prise them out with a screwdriver.
Line the edge connector of the card up with a vacant slot and work out which panel to remove. Remove the panel, press the card into the slot and secure it with the screw that came with it. Put the case back together.