Applying gel liner can be difficult at first.  After practice though, it becomes much easier and gives a quick polished feel to any makeup look.  Here is what I like to do.  The first thing that you should do is get a brush that is a flat tipped brush like this.This is called the laura mercier flat eyeliner brush.  The trick to using this brush is to tap into the product and onto the eye, NOT using the sweeping type of motions that are used with most brushes.

I tap it into the smudge pot and make sure it is fully covered on the tip, then wipe off the excess on the side of the pot, making sure it doesn't look clumpy on the brush. When applying don't use any sliding motions, just lightly tap with the flat brush, then pick up the brush and reposition at the edge of the first tap and tap again...repeat until you have the look you want.  If you want a less defined line, then smooth over it with a q-tip or a smudge brush.