Formative assessment can be a powerful tool to support effective K-12 computer science (CS) instruction. Having teachers and students engage with the formative assessment process can increase student engagement and improve learning outcomes. In this guide, the authors show readers how to apply the five-step process outlined in the corresponding white paper (see ED615714) to systematically develop or select formative assessment tasks and use them to inform instruction. This practice guide illustrates formative assessment literacy in practice. School-level and state-level leaders are encouraged to find ways to promote formative assessment literacy through CS professional development workshops, teacher communities of practice, policy guidelines, and other avenues. These five steps are discussed in greater detail: (1) Define and/or identify fine-grained learning targets to assess; (2) Determine evidence needed to measure progress towards learning targets; (3) Find or create tasks that elicit the desired evidence; (4) Determine how to evaluate and interpret the evidence provided by students; and (5) Relate the interpretation and/or evaluation of the evidence to possible follow-up activities. This practice guide is designed to enhance teachers' understanding of CS standards, how to select and implement appropriate formative assessment tasks, and how to modify instruction to address student challenges identified from the formative assessments.