Over the years last six months (aha, that's when I really got into makeup and nail polish) I've accumalated a few eyeshadows and lipsticks that I simply don't use any more, be it because they are too light, an odd colour or simply weird. So this is what I do with them. Step One:

Find a palette that you don't really use most of the colours. Here's one I got in some set . I use the last four colours because they suit me (gorgeous shimmers and they're dark enough for me) but the rest are too light or not to my taste.
Step Two - identify the unlucky eyeshadow that you want to destroy. Haha, I'm pointing to the one I'm going to use - a brilliant white with silver shimmer. (The nail polish there is a Maybelline Colorama one which I'm going to swatch shortly.)

Step Three - get a bottle of clear nail polish.

These are all the tools you need, plus a small plastic bag. You can do this one of two ways. Either:

a) Scrape out your eyeshadow with the cocktail stick and empty it into a pestle and motar. Grind it finely, then empty it into the plastic bag. Make sure all the pigment is in one corner of the bag, then snip off the end and put it into the clear nail polish.

OR

b) scrape out your eyeshadow into the bag. Use your fingers to crush it then see the last sentance of a).

Perhaps I should have chosen a less suspicious looking eyeshadow, haha.

By crushing eyeshadow in this way you are essentially making a loose pigment of the kind sold by makeup companies like Lime Crime and Aromaleigh. You can then use this to make:

Lip gloss
Blush
Hair colours
Nail polish

It's quite a good way to use up colours you wouldn't use normally :P. How my nail polish turned out:

And a quick swatch:

And there you have it! How to use eyeshadow to make nail polish!
This dries very quickly, but has a matte/satin finish. If you're not a fan of matte/satin finishes then add a topcoat on top.