Choose Convert to To-do… in a note's More menu, on Dashboard or in Notes. The note's title is the To-do's title, and the rest of the note becomes the To-do's notes.
Tick Move the note to Trash in the New To-do window to delete the note once the To-do is made; the message that follows has Restore note. Magpie remembers whether you ticked it for the next note you convert.