To dry herbs in an oven, preheat to between 140 and 160 degrees Fahrenheit. Place the herbs inside in a single layer on large steel cooling racks that allow air to circulate. Leaving the oven door open, rotate the trays every 30 minutes for three to four hours.
The oven door must remain open in order to allow air to circulate. This keeps the air from getting too hot and cooking the herbs instead of drying them. It also keeps the oven from getting too humid. Ideally the oven should be kept at a very low temperature never rising above 180 degrees Fahrenheit. Use a reliable thermometer to keep track of the temperature at all times.