First I took some rolled craft paper and taped 2 strips together in the length and height of the space I wanted it in. Basically the length is the length of my sofa I wanted this over. As you can see I even have a helper with this task...lol. I gave her a piece of tape to play with so she didn't shred my paper. ;)
Then take all your framed pics and place them on the paper and work with them til you are satisfied with your arrangement. I like for mine to not be mirror image sides. In other words the left side is different than the right. But balanced.
Then you trace around each frame with a pencil. After that, pick up one frame at a time and measure very carefully where the nail hangers are on the back. Some of mine didn't have any as the smaller ones are shelf frames, so I measured the center and down from the thickness of the frame. In hindsight I would put 2 leveled nails in the wall for those so they don't keep tipping and you having to straighten them all the time...I'm going to go back and add another nail to mine. Make 'T' marks where the nail should go. Once you've done that for all the frames, move them out of the way. Then use blue painters tape to tape your paper to the wall exactly where you want it. Use a level to make sure they are level. Once satisfied, put nails in all the 'T' marks. Then carefully remove the paper leaving in the nails. If you don't think you can remember the frame set up you have, then use sticky notes to label each nail set to which picture goes there. Once you remove the paper all you will have is a wall full of nails. Then hang your pictures. Done! :)