Painting an even and fine eyeliner is a detail that many find difficult, and I often get questions about just that.
In my opinion, one of the easiest ways is to use a gel eyeliner or a cake eyeliner, and use a separate eyeliner brush. Then you have more control over how much you have on the brush, it feels stable to have in your hand, and a soft brush is easier to get even than, for example, a hard diploma.
And if you have a really good eyeliner brush, it's a thousand times easier than with a worn and bad one, you kind of can not even compare! And many also think that it is yet easier if you have an eyeliner brush with a curved shaft :)
The fact that the shaft is bent means that you can see more clearly where you are painting and also become a little more stable on the hand! I also like it because it is easier to access in the corner of the eye where it can otherwise be difficult to get a nice line with a straight brush.
Nanshy has an awesome curved eyeliner brush that I can recommend to you! Precise Bent Eyeliner Brush is thin and fine, and incredibly easy to paint with.
It costs SEK 99 and is available at Byher.se! :)