The Reef Knot, also known as the Square Knot, got its name through its nautical use of being tied around a reefed sail. While adequate for that purpose isn't a knot to which much tension should be applied; in fact it will reduce the strength of a line in which it's tied by around 55%.
Tied incorrectly it will become the even more inferior Granny Knot,
but remember the well-known mnemonic and you'll get it right every time:
'Left over right and under; right over left and under'.
You'll immediately know if you've made a mistake as a correctly tied reef knot can been seen as two bights cow-hitched together, as shown here.
Remember the mnemonic?
Left over Right and Under;
Right over Left...
Cinch it up tight and you have your finished Reef Knot.