[WARNING!] There seems to be a lot of confusion about using Lemongrass oil for nail fungus. I too decided to try it last summer. I had never used essential oils before and was ignorant about proper dilutions. I applied the oil straight out of the bottle to all my nails on both fingers and toes. I applied it twice a day for less than a month. Then I started to have a reaction: I got a very bad rash and had degloving of my fingertips. I was very discouraged because I saw my nails growing out healthy. I stopped that treatment and am now using a diluted blend of different oils (will report back if this new blend works--so far so good).
However, if you wish to try Lemongrass oil, here is how to Properly Dilute the Essential Oil:
Pick a carrier oil such as coconut or grapeseed oil. Measure out one ounce (30 mL) of your carrier oil and add 12 drops only of the essential oil. This will make a 2% dilution.
Never apply undiluted essential oils to the skin until you are sure you will not have a reaction. For nail fungus, it is common to continue applying the oil treatment until the nail is completely grown out: up to a year. So be sure you are not allergic to the oil before you commit to using it!