Scope
First, you need to define the scope of the problem. Then, what you want to solve with the spreadsheet model and the contents of the input/output data should be clear.
Specify
Once you define the scope of the problem, you should now specify what formulae or logic you will be used when modeling the spreadsheet.
Design And Build
This is the main part where the spreadsheet modeling takes place. You can now design a clear and concise module using the specified logic to solve the problem defined earlier.
Test
Testing the spreadsheet once the modelling is done is important to weed out any margin of errors. Make sure you test the model in many ways to find any inconsistencies or mistakes.