How do I treat my Betta with Methylene Blue?

It states to aid in general disease prevention, detoxification fishes suffering from nitrite poisoning, and to prevent infections in fish eggs, to add 10 drops per gallon. Then it has directions to use it as a dip, which I had never heard of before. So which method do I use if my adult Betta has a fungus disease, and if it's better to dip, how do I go about doing that?