I would suggest first identifying what you find confusing and starting there. What are the features that keep fooling you? The box model? The cascade? Flex axes

I would suggest first identifying what you find confusing and starting there. What are the features that keep fooling you? The box model? The cascade? Flex axes? Specificity? As a declarative language that is really intuitive to start, so simple to get so far, and one which must maintain perfect backwards compatibility with twenty years of web design, so many elements of fundamental functionality are never really studied by casual practitioners.