The sign for yogurt looks like you are making the ASL sign for 'Y', dipping the 'Y' in a tub of yogurt, and then putting it in your mouth with your pinkie finger leading.
First, make your dominant hand into an ASL 'Y' (fist, with pinkie and thumb extended). Make your non-dominant hand look like it is holding a tub of yogurt by extending your finger and curling your thumb round so that it makes a horizontal 'C' shape. Perform the sign by dipping the pinkie from your dominant hand into the imaginary tub of yogurt, and then placing your pinkie near your lips.
Make the yogurtsign as you give your baby a tub of yogurt to snack on.
Teach your baby this sign in conjunction with other healthy breakfast or snack items.