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Newspaper.
Cut newspaper to the size of every frame you plan to hang on the wall. Then tape them where you think you want them on the wall. Take pictures, move, measure spacing, align, and review until you are happy with the groupings and look of the newspaper shapes on the wall.
Then nail away! This saves a lot of heart ache and provides good results every time.
I forgot to take photos of the newspaper when I hung these above the projector shelf, but I wanted to show how this technique can work well in another scenario.