All you need is two contrasting colours of nail polish, and some sellotape.
Paint all of your nails in the same base colour, I needed to use two coats. Then you have to wait for it to dry completely! This is my least favourite part as I have absolutely no patience aha, but it's worth it because if you try to do the next step when they are still wet or tacky, they will be ruined!
So once they are dry, I just cut out some squares of sellotape and had them next to me ready to use. Use the sellotape to outline the triangle shapes so the only part of your nail that isn't covered with tape is the triangle shape.
Then carefully paint over the gap making sure it is all covered, and pull off the sellotape straight away while it is still wet, you will be able to see your lovely neat straight edges :D Repeat on all the nails and that's it! Just a top coat to finish...
I love this design because it looks striking and more complicated than it really is! I think it would also look lovely with spring colours such as pastel pink and mint green, lavender etc. Also, any other geometric shapes/designs can be created using the sellotape method!