Lay the ends of 2 ropes parallel to each other, rounding the ends towards each other.
Make an open loop in the lower of the two ropes. Then pick this rope up and place it on top of the other rope.
Take the end of the bottom rope and thread it back under the upper rope tag end, then back under itself through the middle loop.
Tighten the knot by pulling both ropes with their ends in opposite directions.