Click on a vertex and use the arrow keys to move the vertex around. A sprite will redraw the shape in the new position you did. You can stop moving the vertices by pressing space. To move a vertex again you just click on it. This is the same for all of the vertices. You will know which vertices you are moving when they turn blue. Another cool feature is moving multiple vertices at the same time. To do this, you click on the vertices you want to move then you use the arrow keys to move them.
Thanks to @ianrocks for the idea of using variables for the vertices' positions!
