A poultice is just a fancy word for any moist application put on the skin for healing purposes.
To make a garlic poultice, simply put either crushed raw garlic or garlic powder directly over the body part you wish to use a poultice on. Cover with a clean cloth or bandage. You can also warm the poultice with a hot water bottle to help it penetrate more.
On sensitive skin (such as the face, or on a young child or baby’s skin) use a light coating of oil or cream underneath, like coconut oil or a natural lotion. When garlic goes on the skin, it is absorbed and enters the blood and lymph system (this is how it is able to work and heal!), and you may actually begin to taste it in your mouth! This is just fine, but you might want to chew or sip on something to mask the flavor (mint, parsley, etc.).