I originally posted this as a series of pictures on my Twitter/Mobypicture account, so if you came to my blog via a tweet of mine, you probably have seen this before. But, I figured I'd post the whole thing as a single tutorial up here. As this is going to be quite picture-heavy, you can continue reading after the break :)
Here's all the supplies. A bowl of water, a Q-tip and nail polish remover for the cleanup, toothpicks to draw the marble pattern, and multiple colours of nail polish, preferably high contrast ones so the pattern will really stand out. The colours I used here are China Glaze Midnight Ride and China Glaze White on White. Also used but not pictured is a roll of masking tape.
Oh, and by the way, you can click all of these images for a larger version.
Step 4: Put in your finger, nail side down. In one swift movement, dunk in your finger until the tip is completely beneath the surface. Use the toothpick to shove the remaining polish to the edge of the bowl and come back up.
Step 5: One nail done! The pattern on the water surface has now been transferred to your nail. Gently blow away any air bubbles and let dry. Note how the excess polish got caught on the masking tape instead of your finger.