Sign strong by flexing your arms in front of your body with hands initially touching the front of your shoulder muscles, then move both hands away from your shoulders while closing them into fists, as if showing off how strong your muscles are.
Babies love the idea of fortitude. Teach the sign contextually by allowing your toddler to pick up heavy objects and letting them know they are "strong".
Babies, particularly toddlers, are fascinated by the concept of strength. Use this sign when you pick up heavy objects to provide your baby with examples of strength.