OOP and interfaces in C

Straight of the bat I understand that ANSI C is not an object orientated programming language. I want to learn how to apply a particular oo technique using c.

For example, I want to create several audio effect classes that all have the same function names but different implementations of those functions.

If I was making this in a higher level language I would first write an interface and then implement it.

AudioEffectInterface

-(float) processEffect 



DelayClass

-(float) processEffect

{
 // do delay code

  return result

}

FlangerClass

-(float) processEffect

{
 // do flanger code

  return result

}



-(void) main

{
   effect= new DelayEffect()
   effect.process()

   effect = new FlangerEffect()
   effect.process()


}

How can I achieve such flexibility using C?