Use an old bowl/cup or something similar and fill it with cold/cool water. Drip the nail polishes on the water and use a toothpick to gently swirl the colors. Then dip what ever you're marbleizing into the water. The nail polish will stick to the item based on the pattern in the water. Here's a project created by another Hometalk member that should help you. Wishing you the best.
Here are several projects on the subject:
This is a good YouTube tutorial: https://www.youtube.com/watch?v=lQAk8Md7Wtk
This is a cute idea: https://abeautifulmess.com/2014/05/how-to-marble-with-nail-polish.html
There are good ideas here, too: http://www.pinkstripeysocks.com/2014/07/how-to-marble-with-nail-polish.html