There's a loop at one end, and two strands at the other.

Take one strand, stick it through the loop, and tie a double-knot.  This method makes the necklace adjustable, so you can have it a little loose or tight.

The knots are not hard to undo if you want to take the necklace off, unless you wear it everyday and in the shower without removing it for a while.  The knot gets very tight and might take a while to work out.