How do I add annotations (especially notes) to a PDF in Preview for El Capitan?

I've got a PDF, and I'm trying to annotate it, and I don't understand what to do. It's a student essay and I want to add comments to the PDF.

All I want to do is click on the PDF to indicate a little comment at that point, and then type the comment so I and the student can see it in the sidebar, but I seem to be missing something fundamental to how to make this work (wasn't this easy a couple of versions ago?!).

What's happening now: the PDF is open. I open the Markup toolbar (incidentally, can I have that open by default?). I move the pointer to where I want to add a comment. I press Ctrl-Command-N, and I get an error beep. OK - so I click on the icon marked "Note", and a yellow box appears in the middle of the page. Which is not where I want it. I mouse over it and hold down the mouse to drag it and ... it doesn't move. Instead I'm suddenly drawing a line across the PDF. Now I've just got the little pen nib symbol, and I'm unable to move this note. I scroll the screen down, press Control-Command-N ... and get a beep. Press again and it beeps. Press again and a new note appears, again in the middle. Again, I try to drag the note - but all I seem able to do is draw a line. In the Markup toolbar I see that it's now on the Sketch / "Pen and squiggly line" icon, even though I didn't select that. I click on "Note" again, and presto, there's another note, on top of the one I've been unable to move. So now I've got seven or eight notes, all piled up on top of each other in two different places on the PDF, with no way to move them around.

What on earth am I missing?!

Thanks in advance!