I got a request to make a tutorial on this a while ago (sorry it took so long Nicky!) I honestly forgot this post existed!
The final picture in the collage should be your final product. For some reason, my camera decided to not capture the Aurora Borealis very well for the tutorial. The photo above gives you a better idea of the shade of blue to use.
1. Start with a dark blue base, preferably with some sparkle to it. I used one coat of Black Satin from Rimmel, followed by one coat of Yoga-Ta Get this Blue from OPI.
2. Next, sponge on various colours. In my case, I'm making a blue and purple galaxy. I sponged on (near the centre of my nail) Frostbite by China Glaze, Grey-t to Be Here by Essence, Curaçao by Sephora, Stars In Her Eyes by Ceramic Glaze, Plum Seduction by Revlon, Hello Marshmallow by Essence and Miss Universe by Essence. You don't have to use this many colours. I only did to get the glittery night sky effect I was going for. Basically, you want to lighten the centre of the nail, and add sparkle.
3. Using a white nail polish thinned with polish remover/thinner or watered down white acrylic, start painting the Aurora Borealis. To do this, make an "s" shape from near your cuticle up to the tip of your nail. Go over this shape in certain places to make parts of the Northern Lights pop. In other spots, fade out the white to make more of a glowing look. Just as with galaxy nails, reference photos to make your design more realistic.
4. Finally, using black polish or acrylic, paint a silhouette of trees.
And there you go. An Aurora Borealis manicure!