The double overhand knot is a beefier version of a regular overhand knot. It works well as a stopper, and can also be used to secure the end of the rope after tying another knot.
Step 1: Prepare to Tie
If you're using it as a stopper knot, feed the rope through the intended object or hole. If you're using it to secure another knot, tie the knot, and then hold the loose end of the rope with one hand.
Step 2: Tie the Double Overhand Knot
Form a loop with the end of the rope. Pass the end behind the long end, and through the loop you just formed. Repeat pulling the end through the loop, and tighten the knot.