Well it's actually a bit of an overview rather than detailed step-by-step guide on purpose.

It is important that you have at least basic understanding of the required concepts, like unix filesystem and working on terminal in order not to blow your foot off with this.

Part of this is that you DO have to tune this according to your own device setup. This is because ubiboot is really most useful for developers that want and need to boot different OS'es and kernels easily.