That’s the question. But most people never ask it because they never get started in the first place. Omission requires trying, and therefore failing.
Err is a blessing in disguise. It’s only when you do the work, e.g. write the rough draft, prepare the speech, study for the test, can you go back and revise the approach.
Omission is the act of keeping what is necessary. Not everything is important. Reduction keeps your work succinct. But omission is also what you choose to forget. What’s leftover can be recycled into something else.
Action elucidates the mistakes. Work clarifies what you to omit. To omit is to act, rewind, and to learn.