This might be usefull for Atari 8-bits too, but the space between the Joystick port 1 and port 2 might be different.
 
Firmly insert the switcher into the C64, making sure that it is inserted the whole way and fits firmly without feeling loose. When playing a game, if you encounter a game that expects a joystick inserted into the other port you can use the switches to change the currently active port.
WARNING!
Do NOT move any of the jumpers while the computer is switched on, doing so may cause damage to it. Always power off first!
Do NOT jumper so that more than one button/direction does the same thing, for example: if you jumper the B1 button for UP, then you have to REMOVE the jumper enabling D-PAD UP!
Do NOT use with actual Sega Genesis or Mega Drive controllers, if you want to use one of those - use C64 Mega Switcher instead.
You can use any common Atari-style joystick such as the Competition Pro and Zipstick, they are the ones I've tested with and currently also have in my collection. You can use the secondary fire button if you have one wired with this capability, most commonly the pinout is compatible with the Sega Master System gamepad. An auto-fire circuit has been added to the module, this can be handy if your controller does not include this functionality.