Artemis
Steampunker
To make a perfectly curved line with no jaggies, make sure the "steps" of the line get continuously smaller or larger. For example, the length of the lines in this picture goes 4 > 3 > 2 > 1 > 1 > 2 > 3 > 4Added. Although it could probably use a better explanation than I provided and a smaller picture.
Even when not making a perfectly curved line, it's very important that you draw it with a uniform length of pixels. Jaggies are often caused by certain segments in a line being too short or too long.
(You don't have to add that word for word, that's just how I would explain it)