Drag an icon from the store bar at the top down under the ground to place it. Start with a vertical shaft just below the surface. You can continue the shaft, or add a section that branches off in side tunnels.
Try to get the Gold nuggets without running out of the sections you need, or out of gold.
Work around obstacles, solving the proper path to get everything without running out.
Getting two stars unlocks the next level. Playing that level and not finishing will net you 1 star. Extracting all the gold 2 stars, and all the diamonds 3 stars.
If you want the diamonds, you have to get them before you get the last gold, because the level is over when the gold is gone.