The across sign uses both index fingers while each hand is drawn into a fist. Your non-dominant hand is bent at the elbow, with your index finger pointing straight ahead of you. Then your domimant hand's index finger travels forward, from touching your non-dominant index to about half a foot further ahead, indicating a short distance across something.
Teach your toddler the across sign in the context of locations inside the house, in the neighborhood, or elsewhere, to mark the spot where his or her point of interest is located relative to your own position or another landmark.
This is a useful flash card taught best to toddlers in conjunction with the other prepositions or place flash cards.