Top-level message: it is best to design on paper, and only code afterwards.
Other top-level message: write up your design so others can understand it. Explain how every cell group works, and what it's for. That way, you will have documentation for all who maintain or audit your spreadsheet.
When more precise or concise than natural language, don't be afraid to use mathematical notation. Including Excel formulae.
I shall propose one way to do this, using a particular kind of "pseudo-code". Pseudo-code is notation that looks like a programming language but isn't executable.
With it, I shall build a simple loans spreadsheet. At the end, I shall have two important things: the spreadsheet; and its documentation.[ P | N ]