Selection panels read a great many essays in a short time. That single fact explains most of what makes an essay work, and almost everything that makes one fail.
Answer the question that was asked
The commonest failure is not weak writing. It is an essay that answers a slightly different question — usually the one the applicant had already prepared for something else.
Before you write anything, copy the prompt into a blank document and break it into its parts. "Describe a challenge you have faced and what you learned" is two questions, and an essay that spends nine hundred words on the challenge and forty on what was learned has answered half of it.
A structure that survives fast reading
Open with the specific thing, not the general one. "I have always been passionate about public health" tells a reader nothing they have not read forty times that morning. The clinic, the shortage, the decision you made — that they will remember.
Then: what you did, what happened, what you would do differently, and how this programme fits what comes next. The last part is the one people skip, and it is the part the panel is actually assessing. They are choosing who to invest in, not who has had the most interesting life.
Length, formatting and the boring rules
Stay inside the word count. Panels do enforce it, and going over signals that you either did not read the instructions or did not respect them.
Use paragraphs. A wall of text is skimmed rather than read, and skimming loses you the detail you spent the most time on. Do not use a smaller font to fit more in; it is obvious, and it does not work.
What to cut
Quotations from famous people. Dictionary definitions. Your childhood, unless it is genuinely the reason you are applying. Any sentence that would still be true if somebody else had written it.
The test for every paragraph: could another applicant have written this? If yes, it is doing nothing for you.
Before you send it
Read it aloud. You will hear the sentences that do not work.
Give it to somebody who does not know your field and ask what the essay is about. If they cannot tell you in one sentence, the panel will not be able to either.
Then check the deadline on the organisation's own page, one more time, and note the time zone it is in.