Click on a shape to edit its path. You can grab control points and path segments to move them. Additionally, double-clicking anchor points (red) removes them; new anchor points can be added by double-clicking path segments. Double-clicking control points (blue) locks them with their neighbors. Finally, if you right-click a segment, you can convert it from a line to a curve and vice versa.
You can start drawing your own shapes by clicking in a blank area of the paper. Add straight lines by clicking; curves can be added by clicking-and-dragging. Close the path by clicking the start point, or end drawing in place with a double click. (You can close up unfinished paths later by right-clicking their endpoints.)
Grab a shape to move it. You can change its formatting in the Inspector sidebar. Double-clicking a shape allows you to resize it. You can also remove the shape altogether by pressing Backspace.
Pan the viewing area by scrolling or by grabbing it with Alt key down. Press Esc anytime to hide editor overlays.