Here is three great time savers for you to use every time you mix or master anyone's music. So number one, listen to their reference mixes. These are tracks that they've sent of competing artists so that you can get into the headspace of exactly the kind of sound they're looking to achieve.
Number two, grab a pen, a piece paper, have a play through the track. Take notes of any changes or tweaks you hear upon first listen.
Now, number three, listen through to the track again. Looking at your notes, seeing whether you are overthinking it for the notes that you've taken and see if there's anything else you've heard. So, now you've done those three moves. It's time to mix or master.