Nail polish shouldn't be that runny. If it is, either get some new, or you need to shake it more!
Just don't load the brush up with polish, do a little at a time.
If you mess up you can always use polish remover!
okay, well don't use a small pocket knife. I JUST did, and it closed on my thumb, and I bascially spilt my thumb in half down to my nail. (I have very short nails, I bite them.) And I'm not exaggerating. It's kinda spilt in half. yeh. it burns.
So be VERY CAREFUL.