1
She fell right on her fanny.
Sign "fall" followed by the sign for "fanny" to show exactly where the impact happened. Make sure to use appropriate facial expressions to convey the surprise or pain of the fall.
2
The baby has a cute little fanny.
Sign "baby," "cute," and then "fanny." You can make the tapping motion slightly smaller and gentler to emphasize that you are talking about a small infant.
3
Get off your fanny and help me!
For this common idiom, you can sign "fanny" followed by "get up" and "help." Use a firm, commanding facial expression to match the tone of the sentence.