Eyeliner is meant to give depth to the lashes and make them appear thicker.

If the liner is a bright color or a true pastel, attention will be focused past the lashes to the colored line, as opposed to the more subtle flow of color from dark lashes to dark liner.

Test it on yourself!
Line one eye with a vibrant color, the other eye with brown or black, and see which one looks like it has thicker lashes.

Then, if all my attempts to convince you have failed, and you still prefer to use bright or pastel liners, go for it.

Then draw a line from the inner to outer corner using one fluid stroke, following the curvature of the eyelid.

Do not extend the line past the outer corner of the eye or hug the tear-drop area of the eye.

To start, keep the line as thin as possible, and if a thicker line is desired, repeat the process either across the entire lash-line or simply on the outer third of the lid along the lashes.

Making the line along the eyelid a solid, even one, starting thin at the front third of the lid and becoming slightly thicker at the back third of the lid can be an attractive classic look.

You can line all the way across the eyelid if you like, from the inside corner to the outer edge, or you can stop the line where the lashes stop and start. Along the lower lashes, line only the outer two-thirds of the eye.

Leaving some space on the inside corner of the eye where the lashes end near the tear ducts gives a softer, less severe look.

Plus, wrapping a complete circle of eyeliner around the eye tends to create an eyeglasses look and can make the eyeliner a stronger statement than the eye itself.

How thickly can you line the eye?

As a general rule, for a classic look, the thickness and intensity of the eyeliner is determined by the size of the lid—the larger the eyelid area, the thicker and softer the eyeliner should be.
The smaller the eyelid area, the thinner and more intense the liner should be.
If your lid doesn't show at all, forget lining altogether.