Always snap to grid
Alternatively to the 2d shape editor, you can use this tool to create irregular 2d shapes to extrude into objects that can be subtracted or added later.
Typically, one would use the side view in which to create, for example, a rolling terrain.
Hold 'cntrl' and right-click to create the points, which will automatically link as you go along. Holding the button allows you to move the point, which is important, because the end point cannot be the same as the beginning; one has to click nearby, switch to the left mouse button and move it into position.
Once you have positioned it perfectly (it will snap-to when you're zoomed in)then you can right click (with control) in the centre area and choose 'create brush', subsequently choosing the depth of the extrude of the shape in the popup window.