First tip is to use room temperature water. This is very important because if your water is too warm or cold the polish won't spread the way that it is supposed and therefore won't work.
Second tip is to use a newer bottle of nail polish. Older nail polish tends to separate or become sticky/gloppy. Also the brand of your polish is important. This attempt I used OPI and Essie brand polish and they both marbled easily and stuck to my nails without being watery. Some over polish brands are too watery and don't work as well.
My last tip is to tape around the outside of your nail so that you can speed up the cleanup process. As you can see from the picture below, I used a purple/reddish shade and a gold/reddish shade so when the excess was left around my nail it looked as if I had just murdered someone. After you remove the tape you can dip a q-tip or cotton swab in nail polish remover and clean up whatever is leftover.
So here is my attempt:
This attempt isn't too bad compared to some of my other tries but it still isn't the best. I should have used 2 colors that weren't so close together but because of the whole issue of which brand works, this is what I had to deal with. So hopefully you will learn from my mistakes and end up with beautiful water marbling. If you try the water marbling technique, you can send me pictures on my social media :)
Love you lots <3