As a foreword, although the square knot can be used to tie two ropes together, do not use it for critical loads, because if conditions are not correct, you could end up injuring yourself or others. The square knot is for securing a rope around an object.
Step 1: Prepare to Tie
Find the object that you are planning to secure. Wrap the rope around everything that you plan on holding in place with the knot, and proceed to the next step.
Step 2: Tie the Square Knot
Form a loop with the left half of the rope. Then take the right end of the rope, and pass it through the loop. Feed it around the two strands making up the left half, and back out the loop you inserted it through.
Step 3: Final Word
If you do not have a knot resembling the image above, you may have messed up by accidentally crossing a rope where you weren't supposed to. Make sure to pay close attention, and keep trying until you get it!