First select a line to be the symmetry axis, then a pair of points or edges (of the same type) to be symmetric about the axis. If you want the endpoints of the curves to also be symmetric, you need to add symmetry constraints to them as well.

You may continue to select pairs of points or edges (of the same type) to be symmetric about the axis you already have selected. Use the right mouse button option. Select new symmetry axis when you want to select a new axis. Axis and pairs of points or edges (of the same type) may also be preselected before entering the function.