An accurate financial model will help you to create a best case forecast by testing different scenarios. Your model should be simple and use clear formatting to guide the user, clearly separating and signalling where to find the assumptions, calculations, and outputs. These principles will save you time and, importantly, reduce the risk of error.
You can use Excel to build a robust model that adheres to these principles, but if your model is particularly sizeable or complex, a separate finance or planning, budgeting, and forecasting system might be more appropriate. Regardless of the system you use, it’s essential to plan the design of your model before putting it together, with a constant focus on the overall purpose of your forecast.
For example, if you produce a model to support long-term planning, including integrated profits and losses, balance sheets and cash flow statements, the priority of your model should be flexibility. Alternatively, if you create a budget and operational plan at cost centre level, the focus would be on detail. If you produce a short-term cash flow forecast, where the emphasis is more around speed of updating.
For more tips on increasing the accuracy of your forecasting model, read Phil Gunter-Rees' article: Improving the quality of financial forecasting.