To sign before, hold up both hands open, fingers held together, hands slightly bent, and thumbs sticking out. The non-dominant hand stays motionless in front of the dominant hand. Then move your dominant hand behind your non-dominant, a few inches closer to your body, indicating that something is positioned before something else.
Teach your toddler the before sign in the context of a schedule of events, where you are discussing the preceding activities to be followed by others. This is also useful in the context of locating something at home, where "before" can mean that something is "behind" something else.
This is a useful flash card taught best in conjunction with other preposition or location-related flash cards, but nothing beats a real-life situation.