Since this was my second time putting up a picture wall, I thought I'd share how I did it. I learned a lot after doing it the first time.
.5) Obtain picture frames. I would suggest getting more of each size than you expect to use in case you want to expand in the future or end up using more of one size than expected. You can also optionally get the pictures, and again you might want to get more than you plan on using and then remove some if the shape doesn't fit well.
1) Get a large piece of paper or cardboard. If you have a huge piece of graph paper that would be easiest, but I really liked using cardboard in the end. It should be at least as big as the area you want to put pictures on.
2) (No picture) Figure out how to arrange the pictures. It took Ben and I about three hours to do this the first time. The closer you get the positioning to how you actually want it the better. (For example, don't arrange the pictures with .5" spacing but then plan on putting 1" spacing between them. It'll look different.) TAKE A PICTURE when you're happy with it. I didn't take a picture because I had the old wall picture that I worked from, but I referenced it a lot. So I'll say it again. TAKE A PICTURE OF THE ARRANGEMENT.
3) Draw the nail and picture locations on the cardboard. This also takes a while. Try to find a good place to start from. Maybe somewhere in the middle. Use a square to make sure that your lines are all straight and perpendicular. Having good lines is going to be essential, so take the time to make sure they're square. I started with one picture and then drew lines as required to find out where to put the nail for the next picture in relation to the previous picture(s). Put the pictures back often to make sure everything looks correct. I did the lines in pencil and then made little circles with marker where the cross hairs were for the nails. I also did quick large boxes in marker to show about where the pictures would go, but didn't use the marker lines for any measurements.
This is the end result:
This is a close up. You can see all the pencil lines.
4) Hang the template on the wall. MAKE SURE IT'S LEVEL! We put one nail in and then used a laser level on some of the pencil lines to make sure it was level before putting a second nail in.
5) Put all the nails in the template. Keep in mind that you'll be taking all the nails out to remove the template (although if it paper you might be able to rip it down). The nails don't need to go all the way in.
6) Remove the template and put the nails back in the wall.
7) Hang the pictures! You might want to refer back to your picture of where everything goes or template because it can be hard to tell...there seem to just be a lot of random nails in the wall.