Cut scrap paper to the size of the frame and mark the nail location on the paper. Using painter's tape, move the paper templates around the wall until satisfied. Then hammer the nail through the marks. Remove the paper template and hang!
Do this before making a gallery wall. Hold piece of paper on back of frame and mark with Sharpie where to put the nail-then tape back on wall and nail right through the paper! Voila!
Press painter's tape flat with a putty knife Tape all wall/woodwork seams with painter's tape. Lay the tape onto the woodwork and press it with the corner of your putty knife to prevent the wall paint from bleeding behind the tape onto the woodwork. Let the tape stick out perpendicular to the trim to act as a little protective roof to catch paint drips
Lay out your gallery arrangement. Flip frames over. Mark hanger/nail spot on several pieces of wax paper taped together. Flip waxed paper over, tape to wall, nail into wall where indicated. Remove waxed paper. Hang photos.