Never, ever hang pieces without a guide, especially if you're hanging multiples. Grab some cheap paper, trace the outside of the frame onto the paper with a pencil, then cut it out. Use painter's tape to attach it to the wall. From here, you can move each piece as much as you want without ever damaging your walls! Once you're happy with the layout, find the middle and place a nail. Then, remove the paper from around the nail.
Newspaper is a great option, but I love contractor paper. You can get a roll from your favorite home development store for about $15.
Always hang on a grid. Establish a couple vertical and horizontal lines and stick to them.
Extra tip: It's most pleasing to the eye to use the main piece as a guide. Here, I established a line half way and three quarters of the way down the central image.
Have fun! If you are designing a large wall, consider choosing one style or color palate then working through variations of it. On this wall, you'll see two types of frames, with mats and without mats. Those without mats are printed on metal. Those with mats are printed on paper. You'll also see a variety of medium to dark grays used on the frames. This all creates dimension and visual interest to help pull the viewer in closer while still creating a relaxing flow.