Are you doing this with paper?
If not then I recommend looking into software that will let you develop flow charts....like Visio, or something free like OpenOffice Flowchart.
Presumably your calculator will perform operations like:
((4.7 - 3.1)/16.2)+7.8.
The flow chart would describe the following with symbols:
"add 4.7 to 3.1 then divide by 16.2. Add 7.8 to this result". If you can represent the above with symbols now generalize the expression and draw your final chart.