The biggest thing I did for starters is I went through like the first 10 chapters of Felder & Roussou - Chemical Processes, 2005 edition. As I read, I wrote down key concepts on a pad and made notes within the book. I then attempted to the do every example problem without looking at the answer first. Then I tried to to some problems in the back of the chapter (these problems are typically much harder are tune consuming but, at a minimum, I tried to look at the problem and see if I could find a path to solve it and not worry about the calculations). The book goes over the fundamentals are will jump start you to success with other subjects. I also did the same thing with Separation Principals and Chemical Eng Kinetics.
I also took School of PE online seminar, which helped fill in any gaps.