It helps when you've fed a few that you've made by hand to start in the same thread for it to use as an example (and format), along with one of your resumes. Then copy and paste the job description and have it generate a cover letter for you.
Keep it all in one massive thread, if it makes a mistake, correct it and tell it to apply those changes to future ones as well (in my case it kept saying I had over ten years of experience when in actually I just had ten, so I had to correct that behavior).
Since it's an AI it will sometimes hallucinate, this usually happens if there are terms in the job description that aren't in your resume... either have it regenerate (if it will take more than a few minutes to edit) or strip out the offending sentences. Some will need very little editing, especially if the job description closely aligns with what you have on your resume.
Oh and be polite because it now knows all your skills and can probably murder you in your sleep lol