Make sure the brush is not overloaded with the liquid,pull the outer corner of the eye out towards the hairline(temples),apply the liner in one smooth stroke. Failing that you could try using a solid gel eyeliner with a special eyeliner brush,or one of the liquid eyeliner pens that look like felt tip pens,they are much easier to apply and look just as good.
A table to rest your elbow on, cotton buds to touch up any mistakes and loads of practice. I've been doing it for years and only just get it right now.
As daffy says, though: don't overload the brush, you'll never achieve an nice line, just a row of splodges. :)
I always have my eye open when I apply it.I don't use the traditional liquid ones now though,the gel works better on me now that I am getting older(40) as it doesn't settle into my fine lines like the liquid did.
i always make a right mess of mine
i put quite a bit on in a long thick line and it looks awful but then i get a cotton bud with some lotion on and after its dried wipe off the bits which i dont need and it goes in a perfect straight line