How you do it depends on where you want to put the fan: inside the fridge or outside.
I put one outside on our last trailer. I just mounted a weather proof fan next to the OEM and wired it to the OEM fan. On our present trailer I put two fans inside mounted to the cooling fins, and added a thermostat to control when the fans will run.
They are basically 12 V computer fans: 120 mm. I try to use ones that are water resistant.