You decided you need one. Now work out what actually goes in it, in your own words, without repeating a single line of your resume.
A cover letter earns its place by saying what the resume cannot. If it restates what you did, where and for how long, it has spent the reader attention on something they already had, which is why most letters get skimmed and skipped. There are only a handful of situations where one genuinely adds something.
Five situations where a letter earns its place. Tick the ones that describe you.
One point per row, in your own words. Add rows as you need them, then cut back to three.
Four short paragraphs built around your points, with the rules that decide if it gets read.
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The resume already carries what you did, where, when and for how long. If your letter restates any of that, it has spent the reader's attention on something they already had, and that is why most cover letters get skimmed and then skipped.
There are only a handful of situations where a letter genuinely adds something. Pick the ones that describe you, name the specific points you want to make, and this assembles the shape. It does not write it for you on purpose, because a letter in someone else's voice is worse than no letter, and a hiring manager who reads a lot of them can tell.
Step one
Which of these describe your situation?
Tick all that apply. If none of them do, that is a real answer and usually means the resume can carry this application on its own.
Step two
What do you want the letter to say?
One point per row. Add as many as you need, then cut back to three, because three is roughly what a reader will hold before the letter starts feeling like an argument.