I study the rule book based on my career experience too. Outline like its the bar exam, bold certain lines like you would important case law, and distinguish NFHS from NCAA and ORB where necessary. Worked for me on the bar, it'll get me through umpiring. (Some rec leagues around here use mlb rules, frustrating)
But seriously, I find outlining to be the best strategy. And a lot of the book is unimportant start with the main categories for an ump, for me I concentrate on
1 balks/ illegal pitch
2 obstruction
3 interference
4 dead ball / live ball
5 remedies aka what is awarded by rule and judgement