It's pretty simple. You will need a gauge that reads in 1psi graduations or less, a can of refrigerant, a piercing valve, and possibly an adaptor to attach the hose/gauge to the refrigerant can:
1. Remove the faceplate on the controller to get access to screws that hold it in place.
2. Remove controller and two screws on the bottom of the fridge to get the access panel off.
3. Locate the low pressure side/return line to the compressor.
4. Attach piercing valve to the low pressure/return line. Make sure you install in an orientation that gives you access to attach hose to valve and open/close valve.
5. Attach gauge/hose to piercing valve and purge system of air.
6. Open piercing valve. If the gauge reads less than 2psi you need to add refrigerant.
7. Add refrigerant by turning on the fridge so the compressor kicks on, then open the valve on the can to add refrigerant.
8. Add refrigerant in pulses so that you do not overfill the system.
9. When gauge reads around 2psi close piercing valve, detach hose, and put everything back together.
I watched YouTube videos on how to add refrigerant to large kitchen fridges. It's the same process, just less space.