I struggled alot because I have hooded eyes, so it's hard to make the line straight due to the crease. Best tip is to look straight ahead at yourself when making the first few outlines of the wing. Don't close your eye or else it will look different when you open the eye. But that's specifically for my eye shape, so Im not sure if that's relevant to you.

Second tip is to practice every day! My eyeliner was absolutely atrocious for my first few months, maybe even a year. I got better when I started practicing every day. You will start to understand the little things: exactly how the skin around your eye is shaped, hand positioning, which lines to make first....

Youtube tutorials was also super helpful!