How would police go about arresting a suspected kidnapper?

They would take the girl and the kidnappers to the station seperately, to be questioned. If enough evidence, they would arrest the kidnappers. Of course, the kidnappers would be handcuffed when they are found and brought to the station. The police would have enough probable cause to cuff them and bring them to the station. If enough evidence is had by the police before they find the kidnappers they will arrest them on the spot.