This applied volume of the Vietnamese-language Document Engineering series treats diagrams as a mode of spatial thinking rather than decoration, arguing that the best diagram is the one that best serves the idea, not the prettiest. Across seven chapters it teaches a three-tool funnel chosen by sufficiency: Mermaid for roughly eighty percent of everyday educational needs, D2 for nested containers and more complex layouts, and TikZ for formal publication-quality figures in LaTeX. Chapters cover visual thinking and sketching on paper, minimal Mermaid syntax, seven educational Mermaid patterns, intermediate D2, TikZ for publications, other diagram libraries and tools, and a twelve-week capstone producing five diagrams from paper sketch to final render. Written for learners aged 14 and up, it grounds choices in cognitive load and multimedia learning theory and integrates the file, Markdown, and LaTeX pipelines of earlier volumes.