How do I vent metal doors?
You can buy a louvered vent for the door. You can probably find them at your local home improvement store.

I'm assuming this is a standard hollow core door. The hole can be cut using a a standard jigsaw with a fine tooth metal cutting blade. If it is an orbital jigsaw, you want to set it to have the least aggressive cut as possible to ensure a smooth cut.

Use the vent as a template, and mark the hole using a pencil. Neatness counts here, so try to align it as well as you can to the door. The bottom should be at least a few inches from the bottom to avoid cutting into the metal structure inside the door. Once you have done that, begin by drilling pilot holes near each corner and then cut out the remainder with the jigsaw. After, you can clean up the edges by using a file.

The vent can be attached either with self-tapping sheetmetal screws, or with rivets.