Whenever you are going to apply eyeliner, make sure that your eyes are clean or try to clean your eyes before applying eyeliner.
Take the eyeliner and then start applying eyeliner with the inner line of the eyelashes and after doing this wiggle the liner inside the lashes. This will make your eyelashes look thick and dense.
I personally add a touch of mascara.
In last step, watch your eyes in a mirror to ensure that there are no dissimilarities.