The best way to do it is to use bezier curves, they are rather intuitive. Look up the math on wikipedia. Depending if you just need simple curves or very windy paths you can use more control points.
However drawing good looking curves in C2 is not exactly easy. The best I could come up with is using small sprite sections, calculating the slope at the point in which the section is placed, rotating it to the angle of the slope and so on. Unfortunately you easily end up with hundreds of sprites just for that. That might impact performance. Maybe there is a plugin that does that better.
However if you just want calculate a curve and translate it into a set of tiles constituting a path that you autotile, the whole thing become much simpler. No need for calculating the slope, sprite/object count goes down.