If you're writing your own resume, I'd maybe sit down every six months to a year and think about things you accomplished, and write then down (problem solved, how it was solved, data if possible, impact, etc.). At some point you'll have a list of accomplishments, and you can select the more impressive of them to put on the resume.

(As an aside - this also helps with career stagnation. If you're having a hard time deciding what to write, it's time to move on.

Once you've got some accomplishments, refactor to the most efficient use of words. How can I say everything necessary is as few words as possible? This is where a professional might come in handy if you're not a good writer.

You can run your accomplishments by a friend and say "which two or three are most worthy of a resume inclusion?" - gives insight into what is impressive.

I think that covers most of it. The rest is mostly formatting and summarizing (if you choose to write a summary).