Back is signed by simply holding your dominant hand in a fist while sticking your thumb out. Then use your thumb to point back, over your dominant shoulder, like you are simply pointing to your back.
To sign back, as in, "Going back" to a certain time or place, simply finger-spell the first and last letters of the word: ASL letters 'B' then 'K'. As you sign from 'B' to 'K', move your dominant hand slightly backwards, closer to your body, indicating a return to an older time or place.
Teach your toddler the back sign in context, when they are learning the body parts, or getting dressed, or trying to express where they are feeling something, like a discomfort or pain.
Use back when teaching your toddler about body parts or getting dressed, or in a medical context if necessary.