First, start with a white base. It can be a creme or a shimmer. Also, prep the skin around the nail. Some like to use tape to mask the cuticles and skin, I used chapstik. It's easy to apply and makes cleanup super snappy. It's good for your cuticles too! Bonus!
Next, get your stuff together! Room temp, bottled water will do the trick. I've tried tap water, and for some reason it doesn't work like bottled water does. Have your polishes open and ready for dropping onto the water's surface. I use a dotting tool for my marbling, but you can use a needle or a safety pin or something of the like.
As you drop your polish it should spread out evenly like in this picture. You should be able to tell right away if a polish is going to work or not. If it doesn't rapidly and evenly spread out, I'd advise you to move on to the next color to try. I am telling you, I have tried almost ALL of my polishes to see if they work and there is a very small amount that do. I feel like the aforementioned brands are the best in my limited experience. The more polish you drop onto the surface the higher concentration of color you will have. You can see in this picture I dropped more pink and less teal.
Next, take your dotting tool and drag into the center from the outside. You don't have to do it in a "flower" pattern like this, it's just my habit to do it this way. I've seen some where you drag across to make sort of a Zebra stripe effect.
Then, just dunk! Nail first submerge your nail into the marbled polish, parallel to the water. Before you take your finger out of the water, scoop the polish off the surface with a q-tip. Shake off any water droplets and wait for the polish to dry a bit before you put on a top coat!
Seems simple...really, it just takes a LOT of practice.
Stay tuned for a video tutorial!