Here are a couple of picture tutorials on how to do some gradient nails.
First up, sponging. I prefer to line up the colours that I plan on using on the sponge, dab off some excess polish to help prevent bubbling and then apply them to the nail. Two or three sponging layers will make the polish opaque and of course glitter at the end to help blur everything together or put a top coat on soon after the last layer to make it a smooth transition from light to dark.
Next up, a more time consuming method with a sheer polish. It doesn't have to be a single colour. You can start with a darker colour and then lead up to a lighter colour like this one. Just a bit of polish at a time and slowly going up. Finish off with some glitter or just top coat. I usually put glitter polish on just 'cause it helps to hide any imperfections sometimes.