Step 1: Paint your nails with your base color (in my case, the light grey). Make sure to let the polish dry completely before moving on.
Step 2: Working with one nail at a time, apply a coat of the second polish over one nail. Grab a piece of plastic wrap and move on to Step 3 while the polish is still wet.
Step 3: Immediately blot the top of your nail once or twice with a crumpled piece of plastic wrap. When the sticky wrap touches the wet polish, it will pull up a bit of the color, creating a marbled effect with the base color underneath. If you pull up too much of the wet polish, just add a little more color and dab at it with plastic wrap again. Continue until you’ve completed each nail.
Step 4: If you end up with some polish smudges outside the lines, dip a Q-tip or a small brush in nail polish remover to clean up around the edges. The surface of your nails may be a bit bumpy, so finish your marbled mani with one or two layers of top coat to achieve a smooth and shiny surface.