Gather your materials.

Obviously you can use as many of any color sharpies you want, but I think just black works best. It gives a more clean, finished look and really makes the color of the nail polish pop.

As for the cardboard, I used a peice about two feet tall and a foot wide, but you can use whatever you have lying about. In this case I would say bigger is better though.

After you know what size your cardboard you can sketch on another peice of paper what your design will be. Keep in mind what parts you'll be coloring in with the nail polish. Don't get too carried away, painting with polish is very tedious and fume-y work if the area is too large.

Additonally plan your design accordingly to the colors and amounts of polish you have on hand.

Transfer or sketch or design onto to cardboard with pencil.

Fill in desired areas with polish. You may have to do a couple coats.

After the polish dries go over the rest of the peice in sharpie.

Hang it up and razzle-dazzle! Your wall is now wow-a-riffic!