To add a note to a locked encounter, first go to the encounter for the patient in question. Once there open the encounter context menu (the three vertical dots across from the Subjective label.
From the menu select "Add Post Lock Note" which will open the lock note editor:
Once you have entered the summary and the note details just click "Create Note" and the note will be added to the encounter.
The creation of the lock note will enable a new section on the encounter named "Lock Notes":
An important thing to notice is the "Include Inactive". This allows you to see any lock notes that you may have marked as inactive. To mark a note as inactive just click on the summary for the note which allows you to view the note and mark it active / inactive.