It got a bit out of hand over the years
Have you tried the examples in the library?
The best way to learn is by doing. Read through the examples, play with them by adding a few of your own bits and pieces to see how it all fits together.
There isn't a step-by-step guide for a very good reason. uibuilder itself is mostly a conduit for communicating between Node-RED as a server managing data flows and your front-end web code.
A basic uibuilder flow is 3 nodes long (see the examples) and the basic front-end code is a couple of dozen lines (see the "simple" example).
The skills lie in writing front-end code. uibuilder lets you use standard web development tools. So to learn, you should pick a simple web tutorial for VueJS and follow that.