I can only fit so many details above so i am just going to explain and simplify everything i stated above
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
Some styles are simply just following the shape of your eye shape line slightly and tracing and ending where you desire, but there are also a lot of styles like the one directly above, where you are pretty much just drawing a straight line (straight with a closed eye, and wigging up where you know it looks good, in the one above (just one other style and technique) I prefer to do it the following way so that there is not unfilled space between your lash line and eyeliner line, thickly line your eyes, only following your lash line, do not wig or shape at this point just thickly line your eyes, then starting with a closed eye(do no do this if you cannot close without squinting, it is just the guide for you to see the straight line, start drawing the thin straight diagonal line, wig to where you feel it looks good, if you are just starting i would suggest to start with eyeliner styles that actually have a step by step technique to it, for other styles just practice and get to know you eye shape and what works for them.Have fun goegeouss (gorgeous in a New york or jersey accent) lol