Keep the injured person calm and still. Do not try to straighten the broken bone. If there's bleeding, control it with a clean cloth without pressing directly on the bone. Immobilize the area using a splint or padding to prevent movement—tie it with cloth if needed. Apply ice packs wrapped in cloth to reduce swelling.
Elevate the limb if possible, and seek immediate medical attention. Avoid giving anything to eat or drink if surgery might be needed. Painkillers like paracetamol can help, but never attempt to reset the bone yourself—it can worsen the injury.