Assuming you have a dryer vent made of metal and not brick or another material:
To drill a hole in a dryer vent, you will need to use a power drill with a metal drilling bit. Start by putting on safety goggles and a dust mask. Then, position the drill bit where you want to make the hole and start drilling slowly. Apply moderate pressure as you drill; if the drill bit starts to get too hot, stop and let it cool down for a few minutes before continuing.
Once you've made the hole, clean up any debris inside the vent pipe with a vacuum cleaner. You can then reattach the vent cover or install a new one (if needed).