Really annoyed with myself because I can't figure this out. It involves inserting edge loops to make the corner and snapping the vertices to other vertices.
I have managed it, but can repeatedly get it right, I keep ending up with different (wrong) outcomes like this...

What are the correct key/mouse buttons.
I seem to have tried all option, of clicking, holding and dragging I can think of.
I'm using 2009 if it makes a difference.