This series of blog posts is one option. But my impression is that most of this comes simply with experience. At first the main thing you'd look for is simply whether it uses the indexes you want it to use, or if it does full table scans.
I'd also recommend to simply read the Postgres manual, at least the parts that are not entirely about the syntax of various commands. There is a lot of information in there that will help to understand how Postgres works.
https://explain.depesz.com/
Not entirely necessary when you're more familiar with it, but still useful. And when starting it's much easier to read.
https://www.depesz.com/tag/unexplainable/
This series of blog posts is one option. But my impression is that most of this comes simply with experience. At first the main thing you'd look for is simply whether it uses the indexes you want it to use, or if it does full table scans.
I'd also recommend to simply read the Postgres manual, at least the parts that are not entirely about the syntax of various commands. There is a lot of information in there that will help to understand how Postgres works.