Teach your baby or toddler the difficult sign when you can see that they are struggling with a particular activity, chore, or food item. However, don't stop there. Encourage them to keep trying, with a reassuring tone and a smile. Be sure to also assist them in the given task, and encourage them to mirror you (provided it is really a task or activity suited to their developmental stage: verbally, mentally, and physically).
Teach your child this sign to acknowledge that you see something is difficult for them to do. Then patiently encourage them to mirror you and practice until it becomes easier and more pleasant. Remember to congratulate them, too, for their effort!