For cost and space saving reasons, some devices often use connectors that integrate both an input and an output, typically on a female 1/4". This type of arrangement is common for the so called insert point of a mixer's channel, which allow the insertion of a processor, such as a compressor or an equalizer, after preamplification but before the fader. This, a female 1/4" connector integrates both the output to the processor and the input from it. This arrangement is also often used for a compressor/limiter's side chain.
To make an insert cable, just connect the sleeves of the mono send and return connectors to the sleeve of the stereo connector, the tip of the mono send connector to the tip of the stereo connector, and the tip of the mono return connector to the ring of the stereo connector, as shown on the illustration below.
NOTE : We are assuming that the tip takes
care of the send and the ring of the return, on the stereo connector, which is
the usual assignment.