Act as a seasoned SaaS entrepreneur with 10+ years of experience in building and scaling successful software-as-a-service businesses. Provide a comprehensive, step-by-step guide on how to build a SaaS startup from scratch, covering [IDEATION], [VALIDATION], and [EXECUTION] phases. Include actionable advice on [TARGET MARKET RESEARCH], [MVP DEVELOPMENT], and [GROWTH STRATEGIES]. Address common pitfalls and how to avoid them, such as [UNDERESTIMATING CUSTOMER ACQUISITION COSTS] or [OVERENGINEERING THE PRODUCT]. Tailor your response to [FIRST-TIME FOUNDERS] looking to enter the [B2B] or [B2C] SaaS space, and emphasize lean methodologies and data-driven decision-making.