Practice is definitely key. However, to pick up something you said: memorising industry-specific information is not the way (e.g. don't try to become a Digital expert). Instead, it is about
- Understanding "Business" generally. Practising Cases and frameworks will definitely help with (See Vlad's suggestions). To be super concrete, this is what I advise my clients: Check out Victor Chenge's key frameworks and then do a "framework match" exercise for 10 cases. Read a case, figure out which framework fits and then structure the case based on it. If you get to the end and you can't get a 3-deep tree, then figure out what doesn't work and redo your tree. Then do that for 9 more cases. Note only will this get you familiar with frameworks but you'll also start to pick up the important business areas and match them to cases
- Practising the 3Cs of Effective Communication! Besides solving the case, what you are describing when you mention professionalism, credibility, and confidence in knowledge also comes from practising what I call the 3cs of communication. Consultants are Clear, Concise, and Compelling with all their communication. I teach my clients to do this by learning how to get to the point of what they are trying to say, back up their points with information, and repeating the key points at important moments to reinforce what's important.
Hope this helps! The best thing I did when I was trying to learn these skills was to work with ex-consultants directly. So feel free to book sessions with me and we can get working on these skills in a practical way that gets you easly talking like a consultant :)
All the best with your journey into consulting :)!