depends on your application really. on guitar fx pedals the most widely used method is a 3pdt switch that allows true bypass and a led that indicates when the fx is on, see here:
this method can produce clicks (acoustic noise from the switch as well as discontinuities in the sound if the volume of the fx is much higher then the clean signal for example)
another method is to switch via relays, there are many schematics and examples online, depends on what you want exactly (control fx state by bela or by switch etc.)
the relays method can still produce clicks if volume of clean and fx signal is not the same, so you would need to shortly fade out and fade in the sound when switching fx on and off.