Just click four times anywhere on the screen, these are the control points. A bezier curve will be drawn for you. Now you can click on any of the four points that you have already created and drag it around. Notice how the Bezier curve is redrawn according to the new control points. Toggle the "Polygon" button in order to draw the curve with or without its control polygon and points.
Bezier curves are created by taking a time-varying linear combination of the control points. The Bernstein polynomials are used to calculate this linear combination given by the following equation where Pi is the ith control point:
P(t) = (1-t)^3P0 + 3(1-t)^2tP1 + 3(1-t)t^2P2 + t^3P3 with t running from 0 to 1.