Creative Tank

Create a new symbol and set its type to "Graphic".

Switch to symbol edit mode and draw a shape.

Switch back to "Scene", drag the symbol to the blank space of work area. Then select frame thirty and press F6 to insert a Key frame.

Click on "Add Motion Guide" to form a Guide Layer.

Draw a curve by trying "Pencil Tool".

Select frame one in Timeline, and move the Symbol to the start of line.

Select frame thirty in Timeline, and move the Symbol to the end of line.

Right-click on Onion Skinning and click "Create Motion Tween" to form a tween. (In CS3 or higher version you can click Create Classic Tween.)

If the onion skinning become a light purple background and with a solid line arrow on it, that implies the tween has been made successfully.

You can press "Ctrl+Enter" to check this Flash flick.