To sign yell, take your open, dominant hand, and starting from your chest, extend your hand up diagonally, from the chest to around forehead level, as if to emphasize the raising of your voice. To be emphatic about it, use both hands to signal yell by taking turns taking each hand to and from your chest.
This sign is similar to sing, but there is no need to worry about confusion, as these signs should be taught to your toddler in context, along with the appropriate facial expression.
Teach your toddler the yell sign when you actually want them not to do it, so it would be good to sign don't along with yell, or the contrasting signs of whisper and quiet as well.
Use the yell flash card in classroom settings when teaching toddlers to manage their inside and outside voices.