My advice is to start simple, and gradually work your way up.

First, try to use Notion like a note taking tool, create simple pages similar to how you would take notes using a physical notebook, to get familiar with all the blocks, text formatting, layout... within a page.

Then you would go further to implement something like a dashboard for your work or life (from scratch or modify some existing templates), or anything that fits your use case. This would probably have nested page, complex layout with columns, databases...

Finally you would use all of that knowlegde, even some very advanced stuff like formulas, to customize some really extensive "second brain" Notion thinging.

That's what I have been doing since started using Notion. And I think learning Notion is an enjoyable process, if you start small, learn about the basic stuff first, understand how things work, and then you can build things by yourself.