For self-teaching, one approach is to slowly and carefully work through a good textbook, thinking about how it applies in your daily life, and/or field of activity. One comprehensive text with many questions and examples to work through (with suggested answers to most) is Critical Thinking by Robert H. Ennis, published in 1996 by Prentice Hall, Englewood Cliffs, NJ.
For self teaching, another good way to start is to look at the “TwentyOne Strategies and Tactics for Teaching Critical Thinking” on this web site. Start in particular with the first three suggestions and apply them to questions, issues, conversations, things you read and see, etc., in your daily life, no matter what your principal activities are. Do this frequently and share your thoughts with others.
After a period of time, perhaps a day, a week, a month, or even a year, depending on your present critical thinking level, start applying the other suggestions in “strategies and tactics”. Adapt the strategies and tactics to your own situation. If you are a student, apply them to the subject matter material you face and to your relations with other students and your teachers. If you have a job, apply them to things developing on your job. If you are a parent, apply them to your parental tasks, and community relationships. If you are unemployed, apply them to your efforts to get a job and the issues and conversations you experience.
At the same time, start examining the definition/conception of critical thinking on this site, perhaps starting with the twelve-item “super-streamlined” conception. Does it help you? If not, why not? Perhaps you need to pay attention to some criteria to guide you in making your decisions about what to believe or do. A set of criteria for the various aspect of critical thinking can be found in “The Nature of Critical Thinking: An Outline of Critical Thinking Dispositions and Abilities.” Note the acronyms, RRA, FRISCO, and SEBKUS, and what they mean. Memorize them, and ask yourself, when dealing with your decisions about what to believe or do, whether you are doing what they suggest. But the actual application of the criteria to real situations is not provided by this bare-bones list, so you might first read “Critical Thinking: A Streamlined Conception” to make things more clear. Examples always help.
Sit down and write out an account of your making a decision about some questions or issue, following the FRISCO outline and exhibiting attention to RRA and SEBKUS. Show it to someone and ask for feedback. Revise it and show it to that person again– or someone else for more feedback. Later on, do it again with the same or another question.
For more examples and practice you might work your way through a critical thinking textbook. For a list, see textbooks. Think about how it applies in your daily life, and/or field of activity. One comprehensive text with many questions and examples to work through (with suggested answers to most) is my Critical Thinking(1996) by Prentice Hall, although it is more advanced and difficult than most. Do not rush through the book you choose. Reflect about it.
It would be a good idea, if you can manage, to do all of the above together with one or several others. It really helps to exchange questions and challenges as you proceed. But if that is not possible, try it by yourself.
You can take it from here. Keep applying RRA whenever you get a chance. If you do at least that, your thinking will probably be more critical!