To make the sign for diaper you pinch the fingertips of your first three fingers down by your hips. This would be to symbolize the diaper pins, however when teaching babies to sign we need to keep the signs up by our face because that is where their attention should be when you are teaching them signs.
Another way to sign for diaper is to tap your hip with one hand. This is like a shorthand version for people interested in a simpler sign that may be easier for their baby to learn. Unless you plan on teaching your baby ASL you don't have to sign any particular sign. You can make up your own household vocabulary. It is also possible to teach alternate signs later when your child has more dexterity.
With Blueberry we will be using the first sign, the one demonstrated in the video above.