Both hands make a fist, with your index and middle fingers extended (like the 'U' hand sign in ASL). Your non-dominant hand stays horizontal with the index and middle fingers stretched out. Your dominant hand starts at your nose and turns up and out, then comes back down to meet your non-dominant hand's index and middle fingers together. While doing the sign, your body language and facial expressions should project a fun spirit.
Fun lets you and your baby share a giggle about something. You can help your baby learn the word by making the sign whenever you do something enjoyable together. After going down the slide, you could say, "That was fun!"
This idea is best conveyed experientially. A child will gain full appreciation of the sign for fun if you show the sign when engaging in fun activities such as play, or when attending a party or show.