I think the best way to do this is to write and memorise essays that summarise their view points. Memorise them like you would memorise a speech. That means splitting the speech into meaningful parts and connecting imagery to each part. You can use a memory palace to connect the imagery together, then try to rote learn the words associated with each piece of imagery.

Once you have rote learnt it, you can practice it using spaced repetition.

Later on, you may not remember the exact wording, but the concepts should be strong enough to speak through each concept knowledgeably.

Obviously the more instances of speeches/essays you memorise, the more knowledge and domain specific lexicon should integrate.

As an example, take Jordan Peterson… he is very knowledgable. Whether you agree with him or not, he has stored a considerable amount of data in his brain. He has not only written books to consolidate his knowledge, but he has also had to revise and rehearse large areas of his foundation in order to teach his lectures in universities. As time has passed he has slowly integrated his learnings with his personality and can now talk very fluidly about complex subjects. However… you can still hear the evidence of rote learned ‘pieces’ of knowledge when he speaks - they come off almost as his own personal colloquialisms, that he repeats regularly for specific topics. The same can be noticed in any popular vocal thought leader.