Critical thinking, one of the two most important skills IMHO, is not a defined set of skills but more an undefined set of principles applied in different ways by different fields and, of course, by different people. Probably the best resources, IMHO, are university courses and their cirriculums.

* Most importantly, surround yourself with the very best. As much as possible, read the best thinkers. Stop reading Reddit, blogs, op-eds, Internet rants etc. as much as possible; they will drag you down. If you want to write well, you read the best writers; their high standards and abilities become your norm and you absorb their techniques and ideas; if you read average writers, they become your norm and you absorb average techniques and ideas. It's the same for critical thinking; the best are not perfect - they are not prophets and they don't write scripture; they are flawed and we all should read them that way - but for flawed humans, they build amazing things. Thanks to the wonders of written language and the efforts of endless generations of scholars, you can read some of the very best thinkers over the history of humanity. The cirriculums of university courses can be a great starting point.

* Study with experts: You don't know what you don't know. Amateurs in any field often have enormous blind spots. They ask the wrong questions, based on the wrong assumptions, and have the wrong answers to those questions. There are other critical questions they simply have no idea to ask. Again, expertise shouldn't be taken uncritically, but someone who has spent a lifetime studying the field knows more than you do. The obvious solution here is to take classes, online or in person, or at least utilize some of their work product, their curriculums.

Also, going a step beyond the (brief) question, I'd especially make an effort to learn critical thought from a variety of perspectives and fields, to using variety of tools. Off the top of my head,

* Scientific method, and the philosophy behind it (post-positivism, etc.). Actually designing, implementing, and writing up an experiment is an invaluable experience.

* The difference between the Enlightenment, the birth of modern critical thought, reason and science, and the Middle Ages (and really all of human history) which came before it.

* Quantitative reasoning, including statistics: how they can be applied and misused, and how to think critically about them. I don't mean learning all the quantitative tools (though that can be helpful too).

* Post-modernism, in the critical literary sense: The fact that so many respond so angrily to it should tell you how powerful and challenging it is, as well as the fact that those same people use it without seeming to realize it.

* Anthropology: I've always wanted to study this, as it steps outside the culture, and all cultures, and views them from a different perspective.

* Radical / revolutionary thought: I'm not at all saying to become a radical or revolutionary. But in this field are tools to step outside and think critically about social norms, which is one of the hardest things for humans to do - most people can't even imagine the universe is larger than their norms.

* Historiography: You can't understand history unless you can think critically about the sources of your knowledge of it.

* Economics: This set of tools is very powerful, extremely influential, and widely applicable.

Descriptive knowledge:

* The psychology of perception: All critical thinking begins with inputs of observation and communication, which necessarily are processed through human perception. The psychology of it is both critical and not at all what most people expect.

* The psychology of reasoning and decision making: How people actually make decisions, which is not at all how they think they do.