Incorporating security into the Software Development Life Cycle (SDLC) is crucial for mature companies, yet it is often neglected in the early development stages, resulting in higher costs later. This comprehensive guide explores how to integrate security measures at each SDLC stage, from planning and design through implementation, testing, and maintenance. It emphasizes the importance of training developers in security awareness, using resources like the OWASP Top 10, and employing strategies such as threat modeling and secure code reviews. The guide also highlights the need for dynamic and static application security testing, software composition analysis, and secrets detection to prevent vulnerabilities and breaches. Furthermore, it advocates for ongoing security vigilance through proper infrastructure configuration, incident response planning, and regular dependency maintenance. Cultivating a security-focused culture from the beginning ensures fewer issues as the application matures.