Apply the decoction directly to the affected areas using a cotton ball or clean cloth. Teatree oil is a powerful antiseptic that's very useful against athlete's foot. Dilute the oil with an equal amount of water or vegetable oil and apply it directly to the affected area three times a day using a cotton ball or clean cloth. Just don't ingest it. Like so many other essential plant oils, small amounts of teatree oil, on the order of a few teaspoons, can be fatal.
Camomile oil is also fungicidal.
I would suggest using camomile oil in the same way as teatree oil. Or you can mix the two. If you have hay fever, however, you should use camomile products cautiously. Camomile is a member of the ragweed family, and in some people, it might trigger allergic reactions. The first time you try it, watch your reaction.