hammer to the head is the fastest and most humane. Its absolutely instant. hit it hard and do NOT close your eyes. you may miss and injure worse.
BS #1. Take it to a vet.
Are you a fucking cretin?, Put the poor fucking animal out of it's obvious pain and misery and watch while you do it so that you learn to be more careful with animals in the future, Personaly i'd ban you from being anywhere near an animal again ever!, But seeing as that wont happen it's just a shame that morons like you are allowed within a mile of animals.
Back of the head then wabbit stew or to the grill.
Why dont you off yourself and responders 1 and 4 since you all are a waste.
If you are going to put your rabbit down take her to a veterinarian. First consider if there is a way you could repair her legs, I had a mouse who broke her left front leg and we repaired it at the vet and she lived quite a long time afterward comfortably. This is your responsibility to pay for your pet to be well again because it was your wrong doing that hurt her. You are not a cruel person, it was an accident. I'm sorry.