This paper discusses a companion to the Computer Security Curriculum Guidelines 2017 (CSEC 2017). CSEC 2017 serves both introductory and advanced courses in a post-secondary program. The CSEC Foundations Guidelines focus specifically on the foundations of cybersecurity for introductory courses at the secondary and post-secondary school levels. This paper describes the methodology used to select the foundations, what those foundations are, and how the document is organized. It presents six knowledge areas based on those of CSEC 2017 (specifically, data security, network security, system security, software security, organizational security, and societal security) and makes recommendations about the sequencing and pacing of the material. Then, it provides examples of the learning outcomes for cryptography, access control, and data security topics in the Data Security knowledge area and instructional guidance for their use. The paper concludes with a discussion of the CSEC Foundations Guidelines used by the National Cybersecurity Teaching Academy and Coalition, which offers graduate certificates to US secondary school teachers to prepare them to teach cybersecurity. As a supplement to CSEC 2017, the resulting CSEC Foundations Guidelines were approved by the ACM in Jan 2025.