To sign football, first open both hands and spread all fingers apart. Then bring your hands together by interlocking all fingers, and disengage them afterward. Repeat this lock-unlock sequence a couple of times in front of your body. Think of the players' lines of football as they clash into their competitors.
Teach your child the football sign in the context of watching a game live, on TV, or talking about it based on a story book or cartoon.
This is a useful flash card taught best in conjunction with real-life experience, or when teaching the other sports flash cards.