The square knot is great when you need to connect two pieces of rope; you can always keep adding more, and you can use it almost anywhere. I like to use this one for extending tarps or tying something to my bad. You can until it quickly, but it still stands up well.
STEP BY STEP: Holding the two ropes together at one end, take the left strand under the right and under, then right over left and under. Take the left part of the rope, put it over the right and wrap around. Take the new right side of the rope, put it over the left end and wrap it under.