The bezier tool allows you to create curves with the highest possible precision, by manually selecting the points, and the bezier control points that determine how "curvy" the path is.
After selecting this tool, simply click to create the first point of the curve. Each point you create will be connected to the previously place point in the order they were created. If you press and hold the mouse button down, the point will be placed where you first pressed the button while the position of the control points will be determined by the position of the cursor when you release the mouse button.
You can move any point that's already been placed by dragging it to a new position (the bezier points will maintain the same relative positioning), and after selecting a previously place point (click to select it) you can individually adjust the bezier points also. You may delete a point at any time by right clicking on it to show the context menu, or by using the Remove Points option in the panel to clear all points.
When you have finished creating your curve, you can create an element by selecting one of the drawing tools from the pulldown menu in the Bezier Tool panel (underneath the layer list). Click Create Element to create the new element.