Applying eyeliner has the reputation of being very difficult. Fear not, all that's needed are the right tools and the proper application technique.
My eyeliner of choice would be a gel liner. I use a small flat and extremely thin brush. Holding your eye very taut with the middle finger and looking down into the mirror, I then draw a very thin line from the outside in. This is simply a case of practice makes perfect. When I'm just looking for a very natural (but polished) look, I will simply apply the liner to my entire waterline. Very simple, and you will be amazed at the difference it makes. Just remember that you never want to miss the area that is closet to your eyelashes, and be sure not to overload the brush with product (a little bit goes a very long way) and you can always add more if needed. This will make your lashes look twice as thick. I use and really love Be Pro Gel Eyeliner