Learning how to make dcfs and valuations for the first time, how to get good quickly?

Hello,

I am a rising sophomore who is interested in doing investment banking. I have some accounting background but have never done valuations or made a dcf before. How long does it take and how many dcfs do I need to build out before getting good enough to do great during a potential junior year internship at a BB? Any advice is appreciated as far as the process of learning, resources for studying industry cases for coverage groups, etc. Thanks!