Dowsing is a method for finding ... stuff. There are several different ways of doing it, but the most common way is "crossed sticks".
(I haven't bothered actually doing this - that would be a waste of time for an old sceptic like me - so the images are plundered from the web)
Step 1: What You Need
A wire coat-hanger and two tubes about as long as your closed fist.
You can use anything for the tubes, as long as they are wide enough to allow the coat-hanger wire free movement. Many people use the tubes from empty ball-points, but you can use whatever you want, as long as they allow the coat-hanger wire to fit loosely.
You also need something to cut the coat-hanger, such as heavy wire-cutters, bolt-cutters or a pair of strong snips.
Step 2: Making the Dowsing Rods.
Drop the short parts of the Ls into the tubes, and you're ready to go.
Step 3: How to Dowse.
To start, find a field to find something in. Stand near the edge, relax. Hold a dowsing rod in each hand with the rods pointing forwards.
Walk across the field, followed by your assistant.
As you walk, watch the rods. If your body senses... something, it (apparently) reacts by slightly tensing the back. This tiny movement is amplified by the rods and they move. Whenever the rods make a sudden movement, either crossing or swinging apart, stop and allow you assistant to note or mark the place.
Make several crossings of the field in each direction, noting each point where something happens.
Step 4: And Then?
There's only one way to find out what's under the markers - dig. Make sure you have the land-owner's permission before you do, and beware of what is under your feet: some dowsers claim to be able to find power lines, or you may find an underground river. Or a sewer.
Whatever, if you find something, add a comment with what you found, where and when.
If you keep getting positive results, then maybe you ought to contact James Randi.