The same documents that ship inside the package, readable here without buying anything first.
From an open scene to your first era: Make it Retro, pick a style, tune it in Retro Studio, then what to read next. Read this one first — it is short on purpose.
Everything else: how it works, the camera, eras and the Look, palettes, the Retro shaders and Shader Graph sub graphs, lights, stepped animation and motion, UI, Retro Studio and Doctor, HDRP, performance, limitations and troubleshooting.
Switching and crossfading eras from code, the step clock, the components' public members, and a parameter reference with every setting's field, default and range.
Prefer to try before reading? The live WebGL demo runs the full package in your browser — a survival shooter with a pause menu over every setting of every era, nothing to download. Most questions this manual answers are quicker to answer by playing.