How a note works
Your client opens the link on a phone or a computer, pauses where something needs a change and types what they hear: "vocal a touch louder here", "the snare feels late". The note lands on that second of the waveform, with their name if they gave one.
You see it on the song and on your Notes page. Click it and the song jumps there, so you hear exactly what they mean before you open the session. Reply under it if you need to ask something, and mark it done when the fix is in.
When the revision is ready, upload it as a new version under the same link. The notes from the last round stay with that version, and your client starts the next round on a clean waveform.
The whole round, from upload to approval, is in how to share mixes with clients; every button is explained in Help.