Generate a full detail guide for non-financial founders on how to build a simple financial model for a SaaS business. The guide must simplify a complex topic. It should include: 1. A glossary of essential SaaS metrics (MRR, ARR, Churn, Cohorts). 2. A step-by-step guide to building a simple spreadsheet model that projects revenue based on new customer acquisition and churn. 3. A section on how to model expenses like headcount and marketing spend. 4. How to use the model to run different scenarios. Please include a strong disclaimer to consult with a professional financial modeler or CFO.