There are plenty of tutorials out there on water marbling. It's not a hard technique but it is also not easy to get the beautiful results you see online. It's also a fairly time consuming exercise. I attempted this on four nails but only got a satisfactory result on one so I painted the others in rainbow skittles.
The colors I used here are:
OPI: Red Light District
Happy Go Lucky
Sorry about the cuticles/fingers. I should have washed the acetone off before I took the picture.