Using an infrared (IR) detector to find buried treasure is possible because of the thermal retention differences between regular soil and buried objects. Most famously, IR detectors have been used to find buried bodies, but the same principle of finding cold/hot spots in the ground can be used for buried treasure or other objects. It takes time to cover a large amount of ground with this process, but it is capable of detecting more kinds of materials than metal detectors.
Obtain a map of the location you want to search for treasure. It is very easy to lose track of the area you cover. With the map, draw out grid lines so you can parcel out the search into easily-coverable portions.
Turn on your GPS unit and locate where you are on the map. Mark with a grease pen the time and place. Locate the first grid of land you want to survey and note any distinguishing landmarks that can help orient you if you get lost or turned around.
Turn the IR detector on and wait for it to go through its start-up procedure, if your model of IR detector has one. Look at the monitor on the unit and confirm that it displays a picture of your surroundings.
Point the detector at the ground so it only picks up heat given off by the ground. Never point the IR detector at the sun, as this can burn out the sensors on most models, or at least significantly degrade their abilities. Look at the screen for spots of cooler areas during the day and spots of warm areas at night. Because buried objects are composed of different materials than dirt, they cool or heat at different rates, which are reflected in the different colors on your screen.
Dig down with a shovel until you find the object causing the temperature difference indicated on the screen. In most cases it is just a rock, but in time you eventually find man-made objects.