Make sure both ends of the cords are of equal length, if not, trim ends making sure that the length you have choosen is bigger, (at least an inch & a half) than the circumference of your head.
Place one end of the cord in each of your hands. Put the left cord in between your ring finger and the middle finger of your right hand to make it "stationary". Take the other right cord and wrap it around the stationary cord forming a loop, circling the stationary cord. Wrap the right cord again so you will have two loops close together then put the right end cord through both loops & hold on to the end and slide the loops down the cord until you reach the end forming a knot with the end sticking out of the knot about a 2/16ths of an inch.
Take the stationary cord which now has a loose cord and wrap the cord around the new stationary cord, twice, just like above, put the end of the cord through the two loops and hold the end while sliding the loops down to the end forming another knot with the end hanging out of the knot.
If you still have trouble, try "googling" fisherman's knot.