And avoid getting a Granny Knot (the information below is extracted from Wikipedia and other sources.)
A Reef knot is formed by tying a left-handed overhand knot and then a right-handed overhand knot, or vice versa. A common mnemonic for this procedure is “Right over Left, Left over Right makes a knot both tidy and tight“. Or the reverse.
A Granny knot results when you do two consecutive overhands of the same handedness. The granny knot is a weaker knot than the reef knot.
The working ends of the reef knot must emerge both at the top or both at the bottom, otherwise a Thief knot results.