Mohater covers all the essentials here. One needs to master the basic questions before moving on to more challenging problems. And that is done by studying with solid guidebooks, problem practice, and review.
With that said, Sackmann's Fundamental sets are fantastic. I actually struggled with the OG material the first time. An
Error Log is essential, but my weaknesses were too widespread. I used the Fundamental set, which begins at the most basic and works its way up to about the 600 level. I moved on to the OG 12 and OG Quant afterward and performed much, much better.
Another method that helped me was to reread the
Manhattan books (this time without the end-of-chapter review questions) alongside Sackmann's questions. This heavily reinforced the material.