To sign ahead, begin by making fists with both hands and bringing them together, fist to fist, with thumbs sticking up. Then keep your non-dominant fist in place as your dominant fist moves away, in a half-circular motion, ending up ahead of the other hand by a foot or so. It's like one car overtaking another one on the road and getting ahead.
Teach your child the ahead sign in the context of real-life activities: road trips, picnics or social gatherings where you are explaining that someone went ahead of your family, or that you are going ahead of someone else, and so on.
This is a useful flash card taught best in conjunction with other prepositions of time and location, in preparation for a trip with family, or an activity with friends or classmates. Context is always the best teacher, and the flash cards are only supplementary.