Locate a spot below the garden where the water drains away. Measure out the distance from this spot to the lowest-lying spot in your garden.
Purchase enough 3-inch diameter polyvinyl chloride (PVC) pipe to span the distance that you measured.
Use the shovel to dig a trench between both points. Level the trench so it drops 1 inch for every 10 feet between the garden and the drainage area. Put down about 3 inches of gravel in the trench when you're finished digging it.
Place the piping in the trench over the gravel and cover it with agricultural fabric.
Cover the pipe with 6 inches of gravel and then cover that with soil. The water should begin to drain from the garden and down the new grading you have created.