1)  Using your small screw driver blade, carefully bend back the four metal tabs that hold the brass colored rear case of the potentiometer to the phenolic wafer.  The brass colored rear case is shown at the bottom of the picture after it has been removed.   A knife blade might help pry up the four metal tabs but that's only a rumor which I don't recommend for safety and liability reasons.

2)  If you examine the S-POT1 switch, you will see that it too has a brass colored casing that has been modified to allow the switch to mate with it.  The sidewall of the casing is lower adjacent to the switch's white plastic cam lever.

3)  Rotate the potentiometer shaft so the black plastic pin on the back side of the potentiometer is aligned away from the three solder terminals.

4)  Orient the S-POT1 with the SINGLE section potentiometer as shown in the upper portion of the picture so the lower sidewall of the casing can accept the the three solder terminals on the potentiometer.

5)  Mate the S-POT1 to the potentiometer and hold the two parts together.  There should be no gap between the S-POT1 casing and the phenolic wafer on the potentiometer except where the three terminals are.  If the two parts don't seem to mate, rotate the potentiometer shaft a little until they do mate. Don't worry about where the black plastic pin on the potentiometer ends up inside the switch casing.  As long as that black pin doesn't hit the white plastic cam lever of the switch while mating the two parts, it will go together correctly. 

6)  While holding the two parts together, rotate the potentiometer shaft from stop to stop.  You should hear the switch "click" when it is turned on or off.

7)  Now take your screw driver blade and carefully bend the four tabs on the switch casing so they are flat against the potentiometer's phenolic wafer.

8)  Congratulations!  You have now added a switch to your SINGLE section potentiometer.

NOTE:  The S-POT1 switch was NOT designed to mate with the DOUBLE section potentiometers.  We've had a few "talented" customers who claim to have had success doing this but we have no knowledge of how they did it.  We've never tried mating that combination so don't ask us how we think it was done.  We also do not remember who those "talented" customers were.