Assuming you are talking about a wooden cabinet:
You will want to use a circular saw to cut a hole in the back of the cabinet big enough to fit your venting material. You can use a piece of flexible aluminium ducting or a plastic vent with built in louvres.
Cut a hole in the side of the cabinet near the top using a hole saw. This is where you will attach your vent.
Run the venting material from the hole in the back of the cabinet to the hole in the side. Secure the venting in place with screws or tape.
You may want to put a small piece of wood or foam in the hole in the back of the cabinet to help seal the venting in place.