The notepad is somewhere to jot a thought down before it gets away. It has no title, no Save button and nothing to decide: open it, type, close it. Only you can see it.
Press ⌘K on Mac, or Ctrl+K on Windows, to open the command palette.
Choose Notepad. It sits at the top of the palette under Pinned, so it's there before you type anything. You can also type "note" to find it.
Start typing. The cursor lands at the end of what you've already written.
Choose Back to return to the palette's list.
The notepad saves as you type, including when you close the palette mid-sentence.
The notepad is pinned for everyone to start with. If you'd rather it wasn't:
Open the command palette and highlight Notepad.
Press → to show its actions.
Choose Unpin.
You can still find it by typing "notepad". Pin it again the same way, choosing Pin.