trick for hanging photos, especially in a gallery wall: You simply use the painter’s tape to establish the position of the nails on your wall, use a level to keep them straight and hammer the nail into the right spot.
The question I get asked the most often refers to proper art height. The center of your piece should hang about eye level, which is between 56-60". If you have multiple frames at multiple sizes, cluster them together and determine the middle. That spot should be hung at 56". Always leave 4-8" above a sofa and, like the table chandelier, keep the piece 2/3 the width of the furniture.