Iv found two ways to do this. The first is where you use 'water marbling' to create the tie dye effect. Within this you...
1. Paint on a base coat of your colour or clear nail varnish.
2. Rub Vaseline all around the end of your finger including the cuticles. (this stops the varnish from staining your fingers)
3. Get a shallow bowl and fill with a small amount of water. Add a few drops of your chosen colours, two or three is best.
4. Dip your fingers in the water either one at a time or all together. (leave for around a minute)
5. Take out and put a clear top on to keep everything neat!
1. Blob on the colours your using to your nails.
2. Get a toothpick, and swirl the colours into one another.
3. Cover with a clear topcoat.
Either way seems to work well so give it a try!