What’s the most effective way for one to become an expert in a subject?
The answer depends on the subject matter. For subjects with a “long history of well-established pedagogy” (Chin 2020) (e.g. math, music, and chess): you can use Deliberate practice.
However, that’s not an effective method for developing expertise for other subjects (e.g. business, sales, sport, art, singing, writing, poetry, etc). For these subjects: focus on Purposeful practice and acquiring Tacit knowledge.
Tacit knowledge is the “between-the-lines” know-how. It’s what separates proficient individuals from experts. Traditionally it’s acquired by doing the relevant thing for a long time.
Can we accelerate the acquisition of tacit knowledge? Yes!
Expertise, and tacit knowledge by extension, boils down to pattern matching, specifically Recognition-primed decision making via a mental library of Frames. Improve by working through simulations (Prefer simulations that already exist) or real-world experience (e.g. apprenticeship / on-the-job experience). When necessary, use Applied cognitive task analysis to extract expertise from specific experts.