how to use an insert cable with external processors 

insert cable can also be called a bidirectional cable or a two-way cable.
An insert cable is a type of cable that carries both the input (send) and output (return) signals between two devices, typically in a recording or sound reinforcement setup. This allows for a single cable to carry both the signal being sent to a device (like a microphone or instrument) and the signal being returned from that device (like a microphone preamp or effects processor).
Bidirectional cables are often used in professional audio applications, such as:
Studio recording
Live sound
Broadcast
Post-production
These cables typically have two conductors (wires) for the send and return signals, plus a shield (ground) to reduce noise and interference.
So, "insert cable" and "bidirectional cable" are often used interchangeably, but "insert cable" is more commonly used in the audio industry.