Drive a stake into a spot in the ground with your mallet.
Walk to the other point on the line you want to measure. The longer your tape, the bigger sample you'll get. When you get to the other point, drive in another stake.
Tie your string around the uphill stake. Then walk to the downhill stake and pull the string tight between them.
Attach the string level in the middle of the string. This is easier if someone else helps. Pull the string tight and lift it until you reach level.
Measure the distance from the string to the ground at the downhill stake.
Divide the measurement in Step 5 by the length of the string. This will give you the slope. To write it as a percentage, multiply the answer by 100.