this is SO simple, and really cute. i used a pearly pink colour for the base, and purple for the face. i was just painting little faces on my nails one day, and i noticed that when i turn my hand it becomes sad, then happy, then sad, then happy,them sad, i think you get it:P
First paint your whole nail with the base colour of your choice.
Let this dry.
Once your base is pretty much dry, you can take your toothpick or whatever your using and put a tiny bit of your dark nail polish on the end. then draw the face on.
all the face is, is two dots on either side of your nails. rather far apart. then right in between draw a lowercase "u" without the tail. since there is so much writing descibing the face, it may seem harder, but trust me it isnt, i just wanted to make it clear:)
Now once it it COMPLETELY dry you can put on a top coat if you wish. but make sure it is 100% dry or else it will smudge the face.
Now you have created your very own set of happy/sad kawaii nails:)