For many designers, freehand sketching is the primary tool for conceptualization in the early stage of the design process. However, current education on concept presentation techniques rarely emphasizes the construction of the most fundamental spatial unit, the cube. Incorrect construction of spatial units leads to disproportions that deviate from the common visual experience, and consequently slows down the design process. In addition, weak spatial cognition can be a problem when transferring sketches into orthographic drawings. To address these issues, this research focuses on the training of cube sketching techniques using both freehand skills and digital devices. Based on the station-point approach for two-point perspective drawings, this study developed a computer-assisted instruction (CAI) platform for beginners, with which users can practice freehand sketching for self-improvement. This platform provides instant corrective feedback to every freehand-sketched cube, thus promoting better learning outcome and more effective use of educational resources. Similar platforms have not yet been found in the field. The proposed system's feasibility and practicality were evaluated through a series of teaching experiments. The results indicated that the proposed CAI system can improve students' sketching accuracy by 19% after 6 weeks of instruction, and that the proposed system was found to be more effective than traditional pen-and-pencil instructions in advancing sketching techniques.