I started with short intervals and gradually built up to 90-minute and sometimes longer focus sessions (which I wouldn’t recommend for everyone). For most people, 20 minutes of focused work followed by a 5-minute break is a great entry point. It’s simple, low-tech, and surprisingly effective at training your brain to lock in.
Focus on a topic or multiple topics
Break down it or them into more specific topics
Think about every specific topics
Break down a specific topic into more little specific topics if necessary
Sometime you need to move and combine them
So you just need the better ways of thinking and some tools to help you in the process.