Form a small loop leaving enough rope for the desired loop size. Pass the end of the rope through the loop as though making an overhand knot. Continue around the standing end and then back through the small loop
Pass the end around the main (red) rope to make a Half Hitch. Continue around going over the first turn. Tuck the rope between the standing end and the first turn. Tighten to make it secure (this introduces a slight dog-leg in the main rope). Continue around to add a final Half Hitch.