If you are having trouble drawing your eyeliner freehand then the use of masking tape can help you get the edge of the flick nice and straight. Start by applying your make-up, when you are ready to draw your eyeliner cut and stick a small piece of masking tape to the outer corner of the eyes at the angle you would like your eyeliner to be.
Draw your eyeliner along your top lid continuing to paint onto the masking tape. I like to use cream eyeliners such as MAC Fluidline in Blacktrack, apply cream eyeliners with either a round or small angle brush. Sometimes an angle brush can be easier to use as it already has a straight edge which you can press along the lid rather than the sweeping motion used with a round brush which if you are too heavy handed can drag the skin leading to an uneven line.
When you have finished peel off the masking tape, you should have a nice straight edge. If not then you might not have the tape pressed flat enough against the skin, it can also be quite hard to judge what angle to have the masking tape so if your eyeliner is too low then try again putting the tape at a higher angle.