1.When you close your eye and either look at it with your other eye, or take a picture of it, you can see a natural shadowing, you can see mine above in the first photo, above my lid you see the shadows where i might put my brow bone eye shadow, also there is the same this for your top lash/lid eyeliner some people have this line more than other but you usually can actually see the shape around your eye that you should be almost tracing when you apply your eye shadows and eye liner i see some people who have a clear perfect eyeliner line without eyeliner on and i will say to my self ''wow she must have had an easy time learning her liquid eyeliner lol, and this is also the reason i think is why some people have an easier time with it and some people have a harder time with it, some people have a super obvious line and some people it isn't so clear, but basically whether your natural line is clear or not, it is basically just where the straight top line(with a closed eye,) leads, and where the bottom lash line meets up.
This is only one technique that i find works for me, and this is only for one style of eyeliner, you will notice a lot of people do it much differently from on an other, some do with a thinner wigged line, or a longer line, but the ones you will notice are most different from this technique is something like below