You can add a note to a lead at any time in that lead's Lead Profile window. Just click the Leave a Note button: This will open a form in which you can write your note about the lead. Notes saved here will be collected in the Notes tab of the Lead Profile.
It's important to understand that leaving a note does not update the lead's deal status nor communicate progress nor events to the system — you should still always use the Action and Phase Action buttons in the Lead Profile for that. The notes tool is merely intended to aid you in retention of significant details about the lead. For example: "A three-car garage is an absolute must-have for Jane." or "John's daughter just had twins." Another good use of notes is as a supplement to snoozing, to help you remember why you set the snooze.
Be aware that all assigned agents will see your notes.