The Pencil tool draws a freehand line on the image. There are a number of optional settings that change the way the Pencil applies the effect.
To draw a line:
Click the button.
In the Tool Options, select the Line Width. Selecting Additive Blending is optional.
Select a color with the Color Picker.
Select a Blend Mode.
Select an Opacity.
Hold the mouse button and drag the pointer across the image to draw the line.
The right mouse button draws a different colored line based upon the secondary color chosen in the Color Picker.