There are no rules for achieving the perfect liner, only little tricks and patience to help you work while you are not completely satisfied with the results. My first recommendation would be - always apply liquid liner in front of the mirror while standing (or sitting) in your own plane. It is very important to apply the liner while watching straight at yourself, because it is the way to achieve symmetry on the face and the eyes. If you have drooping eyelids, or wrinkles, feel free to gently lift extra skin with finger or a brush.

Be sure to have a firm support, resting your hand on top of the table for precise application.

Simplify your job, take eyeliner pencil and draw out the desired shape. Do it by adding dots or simply straight lines. For this you can use a black eyeliner over which you will later pass the liquid or gel eyeliner. After you draw a line, and connect them so you will achieve the desired shape.

If you want to create cateye look, line in the pattern with a pencil and a way to start is from the outer edge of the eye in the plane of the lower eyelid to the outer edge of the eyelids. If you have a habit of filling in brows, extend them prior to eyeliner application. This way your eyeliner will not be short or asymmetrical. Elongated line is connected with a line that runs next to the eyelashes of the upper eyelid.

Cat-eye can have a small, curved end or it can extend far beyond the outer edge of the eye for the dramatic look. In any case, it is necessary to get a perfect line. Do not despair if you cannot achieve it only by drawing. My recommendation for debugging the whole liner thing is micellar water combined with Q-tip. It dries down quickly, will remove the liner and not leave a greasy residue, so you can restart in a short time and go over the same place.