Education Plus Development, offers a six-part blog series on teaching 21st century skills, including critical thinking, problem solving, metacognition, and collaboration 

https://www.brookings.edu/blog/education-plus-development/2017/10/17/how-do-we-teach-21st-century-skills-in-classrooms/

Another website that provides a comprehensive guide to 21st century skills is Panoramaed, which lists critical thinking, communication skills, creativity, problem solving, and more as the most commonly cited 21st century skills https://www.panoramaed.com/blog/comprehensive-guide-21st-century-skills

For a more in-depth understanding of the competencies encompassed by the 21st century skillset, the article published in the Journal of Learning Sciences provides a review of frameworks that include critical thinking, problem solving, digital and technological literacy, and global awareness, among others https://journals.sagepub.com/doi/full/10.1177/1745499919829214