Pillow is signed by leaning your head slightly towards the direction of your dominant hand, as if you are laying your head down to sleep. Then take your two open palms, facing each other but some distance apart, as if there is an invisible pillow in between both your hands. Make a fluffing motion with both hands as your head remains tilted to the side of the "pillow".
Teach this sign to your baby when he is about to be tucked in. It's also good to teach pillow along with other signs related to sleeping and the bedroom.
This sign is best taught when there is an actual fluffy pillow around, rather than with just the flash card. It's always best to teach your baby the signs with their real-life counterparts, as the multi-sensory experience lends to better memory of the sign.