Consider this
Besides that the old chip could be simply replaced by the micro itself, to create the flow diagram, you could try this and see how far you come:
Put in writing, yes, in writing, every single step you think is needed to make your circuit work.
You will make some mistakes, maybe, going too much into details or by the contrary, overlooking the minimal ones.
With that excercise you will start creating in your mind the base for a flow diagram.
When I start something that is completely new to me, I do just that. From there to a proper flow diagram there is not too much else to be done.
Instead of asking for it, put your efforts first.
My bet is that way you could get concrete help.