Using unplugged coding activities to promote computational thinking (CT) among secondary learners has become increasingly popular. Benefits of using unplugged coding activities involve the cost-effective implementation, the ability to promote computer science concepts and self-efficacy in learning computer programming, and the engaging nature of active learning through collaboration. However, there is insufficient information regarding qualitative investigation on how learners develop their CT skills while working on unplugged coding tasks. This study therefore developed unplugged coding activities using flowcharts for high school students to learn computer science concepts, and to promote their CT skills. The activities consisted of five missions encompassing the concepts of sequence, repetition, input & variable, condition, and loop with condition. The data collection was carried out with 120 high students whose participation was video recorded and observed. A thematic analysis revealed that patterns of CT development started from initially developed, to partially developed and fully developed stages, respectively. The various stages were derived from different abilities to apply the computer science concepts to complete the missions with different expressions of CT skills. In addition, the study proposed a 3S self-directed learning approach for fostering the CT development, composing of self-check (in pairs), self-debug (in pairs), and scaffolding. It is therefore suggested to use the 3S model integrated with the unplugged coding activities for developing CT among high school learners.