You're in luck! The adorable Marissa from Roost Home figured out a quick, easy and exact method for hanging a gallery wall. Go here for in-depth instructions, but the gist is...
1. Trace your arrangement on wax paper, making sure to mark the hanging mechanism.
2. Tape the wax paper template on the wall and tap in your nails.
3. Hang up your art and tear down your wax paper (which ever you feel comfortable doing first)
Voila! Easy and exact! Thanks, Marissa!