What is the best way to cure a nail fungus underneath my toenail?

Several treatments. The reason you hear so many different treatments for toenail fungus is because there is no perfect, sure-fire treatment that works 100% of the time. I've seen vaporub work, as well as other natural oil remedies, but rarely. You can also try laser therapy, antifungal creams, paints, or even oral medications if your doctor deems you healthy enough to take them.
See below. Trim the nail back as far as you can and start applying an antifungal liquid to the nail and nail bed.