Adding Freon to a Toyota Highlander is a relatively easy process, and one that can be done by most people with basic automotive knowledge. To begin, you should make sure your vehicle is parked in an open area where there are no other cars around. Then, locate the air conditioner system's service port; on the Toyota Highlander this will usually be located near the firewall or fender well of the engine compartment. Using a refrigerant gauge set (available at most auto parts stores), connect it to the service port then turn on the car’s engine and let it idle for 10-15 minutes so that all components related to air conditioning have time to warm up before taking readings. Once complete, take two readings from both sides of your refrigerant gauge set - one reading should indicate how much Freon is still left in your AC system while the second indicates how much pressure remains inside your AC lines. If either of these readings falls below factory specifications then adding more Freon would be necessary - simply attach a new canister of R134A refrigerant to your refrigerant hose using an adapter then slowly add additional freon until you reach optimal performance levels as indicated on your gauges. Lastly, shut off any valves connected between hoses and disconnect all connections once finished including those associated with any adapters used during installation – this will help ensure safe operation throughout use!