In order for your castanets to produce the desired sound for "golpes" (strikes) and "carretillas" (rolls), it's important that they be tied properly. Castanets are worn -- one on each hand -- so that the string fits over either side of the knuckle of the thumb, with the "oreja" (ear) of the castanet braced against the inside of the joint. A slipknot is made so that the string can be adjusted quickly and easily. When this knot is tied correctly, pulling one end of the string will tighten the castanet and pulling the other end will loosen it.
Place both halves of the castanet together in the closed position with the ears on top. Lace each end of your string through one side of the matched holes, so that both ends of the string are pointing away from you. Adjust the string so that the ends are the same length.
Hold the castanet in your right hand and, with the thumb and index finger of your left hand, hold the two ends of the string side by side, pinched off at approximately three finger widths away from the castanet. Transfer the castanet to your left hand, holding it by the pinched strings.
With your right hand, loop the left string in front of the right string, passing the end counterclockwise around the right string and bringing it back toward you, through the loop. Pull the knot in the left string tight.
Put the castanet on your thumb and adjust the string as needed. Make another knot about an inch away from the first on the right (sliding) end of the string. Trim both ends so that no more than 1 1/2 inches remain.
Repeat steps 1 through 4 for the other castanet.
Knotting the end of the right (sliding) string will prevent it from coming out of the slipknot. Old-fashioned Venetian blind cording makes exceptional castanet string.