Put the rectangular sensor (the one with the long cable) on the tree. The spike should point towards the roots.
Connect the BNC connector to the (black) amplifier box.
Connect the soil sensor (cylindrical) to the amplifer box.
Tie a loose rope around the tree. The rope will help you to keep a constant distance between the tree and the soil sensor. Move the soil sensor around the tree by small steps, making sure that the rope remains tightened.
Make a measurement at each sensor position.