How to become strong at technicals and modelling?

I have been interning in IB and VC and will now start a PE internship. However, I feel as if I am not very good at the technicals. During my internships, I did not really get to do anything technical but instead worked on marketing materials/market analysis/internal stuff. I have taken some finance courses and modelling courses so I know the basics and can build a decent 3-statement model. However, I would like to better understand cash flows, the balance sheet and how to model it. 

For example, I would like to understand which cash flows to look at for different companies and why certain adjustments are made. I would also like to understand how to model items on the balance sheet such as deferred taxes, right-of-use assets, lease liabilities, depreciation periods for existing assets etc. I find that most modelling courses simplify the balance sheet and if they do not, they give you the assumptions instead of explaining how to make them yourself. 

How would you go about learning technicals such as the ones I've mentioned above?