I do what Val stated with the 50/50 mix spray and adding it in the water bowls.ACV has a bunch of health benefits not only for dogs but for people too. It can be used both externally and internally for different things.
Externally you can use it as a 50/50 mix with water in a spray bottle and apply it that way or do a mix and bathe them in it. Internally it provides even more benefits, you can easily mix it into their food or water. I personally mix it into my dogs' food directly.
Just a sampling of the benefits of ACV (preferably you want ACV with the "mother" in it): it's antibacterial and anti fungal; it improves the immune system, liver, bladder and kidney function; helps with itchy skin and allergies; balances digestive enzymes; normalizes the pH of the body; repels both internal and external parasites like fleas and worms; provides potassium for tissue and joint health; ...the list could go on and on!
For fungus you could do a 50/50 mix of ACV and water, and it would be up to you if you prefer to use it in a spray bottle and spray it on the affected area or you could use it as a dip.