Fresh crushed Garlic is usually preferred since the Allicin molecule (created in the crushing) only has about a 5 minute life span. This is not to say that Garlic oil will not work... but it is better fresh as well.
We lightly crush a couple Garlic cloves, peal the skins off, then we put the pealed cloves on a paper towel. We fold the paper towel over the garlic and crush them with the bottom of a jar or a pan or something once they are in the paper towel. Then we run water as hot as it comes out of the tap into a cup and dip the paper towel with garlic in the water. Squeeze lightly, and apply directly to the chest, feet or back.
It can and will burn (as has been mentioned) so depending on the strength of the Garlic and sensitivity of the skin I leave it on for between 45 and 90 seconds.
I then "re-dip" the pad in hot water and re-apply to another surface... then I toss the pad in the trash.
Warning: take out the garbage! Old Garlic pads will make your house smell awful!