Start with the question the model must answer.
A model for a funding round may need a different structure from one used to approve a new branch or assess an acquisition. Define the decision, intended users, reporting periods and outputs before building spreadsheet tabs. Explain who will maintain it and what information they can obtain reliably.
Make the drivers visible.
Revenue can be built from customers, volumes, pricing, contracts or capacity. Costs can follow headcount, activity or specific commitments. Document the source of each significant assumption and distinguish evidence, management judgement and unresolved inputs. Avoid hard-coding assumptions inside calculations.
Connect the financial consequences.
Where appropriate, the scope includes profit and loss, balance sheet and cash flow, with schedules for working capital, investment and financing. The model should show why profit and cash differ and how a change in one assumption affects the wider forecast.
Build for use after the project.
Agree an editable Excel or Google Sheets deliverable as appropriate, with an assumptions guide, checks, scenario outputs and a handover. A useful model needs understandable update steps, not just a polished output on the day it is delivered. Maintenance or further revisions can be separately scoped.