Fill your washing machine with one load of towels.
Pour 1 cup of white vinegar either directly onto the towels or in the detergent spot. DO NOT USE DETERGENT OR FABRIC SOFTENER.
Turn the temperature up as high as it will go.
Leave towels in the washer.
Pour 1/2 cup of Baking Soda onto the towels. DO NOT USE DETERGENT OR FABRIC SOFTENER.
Again, turn the temperature up as high as it will go.
Dry towels normally.
Your towels will not smell like vinegar. Actually, they won't have a smell at all unless you use a dryer sheet in the dryer.
I would suggest doing this every other month, or as often as you prefer. I do mine whenever they start to get a strong detergent or other smell to them, which is normally only every other month.
Have a fresh day! :)