I have practiced water marble nails a few times, and they are quite hard. This was probably the third or fourth time I did them, and they turned out much better!
What you do for this style of nails is take a small glass of cold water, 3-4 colors of nail polish, some sort of tooth pick, and tape. You first start by taping off your finger with clear scotch tape so only the nail is open. You then quickly start dripping a drip of nail polish into the cup, and follow it with the other colors of your choice in a bulls eye fashion. After you put as much nail polish in the cup you then dip your nail into the water slowly and use a toothpick to wipe away the extra nail polish in the cup.
After you take the tape off your fingers there may be some nail polish that got in the tape and on your skin. If that happens simply wipe it off with a bit of nail polish remover on a cotton ball.
The final result should look like this! :)