Organ-on-a-chip (OoC) has emerged as a revolutionary technique in recent decades, capable of replicating essential aspects of physiological and pathophysiological processes of human organs in vitro. Serving as an effective tissue culture method for creating digital twins, OoCs show significant promise and have found applications in disease modelling, drug screening, and tissue engineering. However, there has been a lack of emphasis on the fundamental design principles of OoCs in existing literature, a crucial aspect that cannot be overlooked, especially for beginners venturing into the realm of OoCs. Therefore, this paper endeavors to provide a comprehensive overview by delving into the historical development of OoCs, outlining the characteristics of their scaffolds, presenting design strategies for both conceptualisation and fabrication processes, and offering a detailed description of design mechanisms and guidelines based on recent research publications. Furthermore, it explores future prospects and challenges within the OoC domain. Serving as a foundational guide for those new to OoC exploration, this paper aims to furnish a thorough introduction to the fabrication and design strategies employed in OoCs.