First off, check to see if there are any brass shims under your nut. If there is, remove it and try your guitar without the shims. Otherwise, you'll have to take off the nut and file the wood under your nut. Unless you got a way to file down hardened steel, filing the wood under the nut is your best option.
By the way, what's wrong with your guitar that makes you want to lower your
nut height?
Guitar playing too stiff at the nut?
