Inkscape is a vector graphics program that allows you to create pictures by defining individual components -- such as lines, circles and squares -- instead of altering a blank canvas. In Inkscape, lines are defined by a series of nodes. Curved lines, also called "Bezier curves," are lines that contain four points: two end nodes that define the line and two points that define the curve itself. These are called "handles" in Inkscape. You can create these curves by defining the points and the handles, or you can use the Freehand tool to draw a curve using the mouse.
Click the Bezier Curve tool button on the left side of the screen. This button has an icon of a thin, blue ballpoint pen. When you mouse over the button, the tooltip should read "Draw Bezier Curve" or "Draw Bezier Curves and Straight Lines."
Click somewhere in the Inkscape canvas to place the first end node.
Click elsewhere in the canvas to place the second end node. Press "Enter" to finalise the endpoints.
Click the "Edit Path" button on the left side, which displays a picture of a black triangle and a curved line with three nodes.
Click and drag the line you created earlier to alter the curve. To individually control the curve associated with each endpoint, click one of the nodes -- the small grey diamonds -- and the line's handles will appear. The handles look like empty circles with a light-blue line that connects them to a node. Click and drag the handles to change the curve of the line.
Click the Freehand tool button on the left side of the Inkscape window. This button shows a yellow pencil drawing a curved line.
Set the "Smoothing" value for the tool at the top of the screen. This value is a number from 1 to 100 that determines how closely the program follows your motions. At a value of "1", the line you create will be almost exactly the same as what you draw on the screen, and the resulting line may contain hundreds of line segments. At higher values, the program smooths the curves and decreases the total number of line segments.
Click and hold the mouse button on the canvas to draw the curve. Release the button to stop drawing the curve.
Adjust the curve using the Edit Path tool on the left side of the window. Click and drag the nodes to move them. Click on a node to change its handles.