Hammer in a peg level with the soil at the top of the slope, and hammer in a second peg three feet (one meter) farther down the slope. Lay the wood on the pegs and, using a level (lay the level on top of the plank), adjust the lower peg until the wood is level.
Remove the wood, and measure the height of the second peg from ground level. This tells you the “fall” over three feet (one meter).
Leave the second peg where it is but hammer it in level with the ground. Hammer a third peg three feet (one meter) farther down the slope, lay the wood on top, and again adjust the height of this lower peg until the wood is sitting level. Then measure the peg, as you did in Step 2.
Repeat Step 3 until you have measured the whole slope. To calculate the overall fall of the gradient, add up your measurements.