First principles. Ask what a model is for and the layout designs itself. A model takes things a human chooses (assumptions: a growth rate, a tax rate, an opening balance), runs them through machinery (formulas), and produces things a human reads (outputs: a valuation, a cash balance, a covenant ratio). Those are three fundamentally different kinds of cell, and the professional rule is brutally simple: never mix them in the same region. Inputs live in one clearly labelled block (ideally its own sheet or a fenced top-of-sheet zone). Calculations live in another. Outputs (the summary a reader actually wants) live in a third, usually the first sheet you see when the file opens.
Why is this not just tidiness? Because auditability is a function of separation. If assumptions are scattered (a discount rate typed into a DCF formula here, a margin buried in a revenue line there) then to change one assumption you must hunt through formulas, and to check the model a reviewer must reverse-engineer which numbers were choices and which were consequences. Separate the three and the model becomes drivable: every lever a user may pull is in one place, coloured so it's obvious, and everything else is a formula that should never be touched by hand. Damodaran's spreadsheets at pages.stern.nyu.edu/~adamodar open on an inputs sheet for exactly this reason: the story of the valuation is "here is what I assumed," and the layout tells that story before you read a formula.
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How should you lay out the sheets in a financial model?
One role per sheet. The generalisation of the three-block idea is that each sheet should do one job, an Assumptions sheet, a Calcs/Model sheet, an Output/Summary sheet, perhaps a Data sheet for pasted source data. A sheet that is half assumptions and half a chart and half a scratch calculation is a sheet no one can hand off. When a model grows, you add sheets by role, not by cramming. Flow runs consistently: top-to-bottom and left-to-right, time running across columns the same way on every sheet (Year 1 in the same column everywhere), so the eye and the fill-handle both move predictably.
UI and keyboard. Rename a sheet by double-clicking its tab, or Alt, O, H, R. Colour a tab (a light way to signal role, inputs one colour, calcs another) via right-click ▸ Tab Color, or Alt, H, O, T. Jump between sheets with Ctrl+Page Up/Ctrl+Page Down. Group your inputs visually with a border and a fill, and, critically, label every input with its units (₹ lakh, %, ×, days) in an adjacent cell, because an unlabelled 12 is a bug waiting to happen.