I've read as much as I can from too many internet sources tonight and got even more unsure.
Please can anyone here with relevant technical experience
provide a definitive answer to this question ?

Re. "Y" format insert cables, 1 stereo jack and 2 mono jacks;
does it matter which mono jack plugs into either the input or output of any FX processor.

I've been told I need to buy some of these cables, but I'm worried in case I connect them the wrong way round and blow something up.