Advice for single author Physical Review Letters submission

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Hi all, I’m a fourth year PhD student in chemical physics and I’m about to submit my first single author paper to PRL. I have multiple first author papers by now including one in Science Advances and one in PNAS. My PhD advisor is a big shot in the field and this time he’s convincing me to do a single author paper without him as I’m about to graduate.
If there is someone else who has had a similar experience, are there any advices for the submission and how to approach the cover letter? Also, this will be my first PRL submission so i would appreciate some insight on the difficulty, overall timeline and any specific tips.