Click on the map to add Polygon markers, click on a marker to remove it. When you've outlined the polygon you want, click Draw polygon*.
 
Clear all |  Draw polygon |  Hide / Show markers |  Change color |  Show center
*You can add markers, remove them, add some more, and then render the polygon again, but the order in which you added the markers counts, so the results might not always be what you expect!

Polygon javascript code