The french fries sign looks like you are holding a french fry and dipping it in two tubs of sauce. Take your thumb and index finger on your dominant hand and touch them together, forming the letter 'F' in ASL. Then pivoting from your wrist, move your 'F' hand in succeeding dips or nods, first to the left and then the right.
Most babies are very interested in the words associated with different foods. Somehow they very quickly learn words associated with favored treats like french fries, even if they are only exposed to the food and the sign a few times. Why can't they pick up on putting their toys away, or algebra, so quickly! Learning favorite foods like french fries also gives your baby a way to tell you what they want instead of throwing a tantrum. Even if you don't accommodate the request, just acknowledging it and making your baby feel heard will reduce frustration.
Once your baby discovers the world of eating out, french fries may just become one of their favorites. Fries are also good to teach babies concepts like counting and shapes. Use the fries to make geometrical figures like square or triangle, or tell your baby to pick up three fries.