Press (S)quare, (C)ircle, (R)ectangle and (T)riangle to spawn shapes at the mouse cursor. Right Click to remove a shape. (H)ide to hide the in-game instructions. 0, 1, 2 & 3 to load presets.
You can drag objects with the left mouse button. (This is buggy, if you stop moving the mouse, it stops holding the object, regardless of if you are still holding the mouse button down)