First of all, it's important to tell your story, not repeat your CV!
I would generally approach it like this:
1st paragraph (short): Explain what you're applying for and why, ideally mentioning who in the firm helped you realize that it's the best fit (referral or other friendly contact)
2nd paragraph (main section): Tell them who you are by sharing what you like and care about, which you should illustrate via a key experience or the rough evolution of your CV (be careful to not repeat your CV but rather narrate the drivers behind your decisions). There's no guidance as to what's in/out of scope and you are in control of the narrative.
3rd paragraph (short): Tell them why you and the target firm are a match made in heaven and why your trajectory shown in the previous section naturally led you to this point.
4th paragraph (1 sentence): Thank them for their time and consideration and tell them how excited you are to hear from them!
Writing cover letters and storytelling is a passion of mine and cannot be underestimated in its impact when done well (or poorly for that matter). Feel free to get in touch if you're seeking help with your letter!