Rectangle is signed by taking both pointer fingers (while the rest of each hand is drawn into a fist), then starting from the top center of the invisible shape, you make parallel, mirror actions as you trace the outline of a rectangle from either side, top to bottom.
Teach your baby the sign for rectangle whenever you encounter an object or image possessing that shape. It's also useful to teach your baby this sign in conjunction with the other shape signs: triangle, circle, and square. As your child gets the notion of each shape, see their faces light up when they are able to match objects with their corresponding shape. Make a game of this simple geometry lesson!
The flash cards a are a great medium to teach the basic shapes to your baby. Once they learn to make the association between rectangle and the flash card diagram, try and help them broaden their understanding by finding rectangles in your environment. For example, they may see rectangles on the table or objects.