We will be guided in our pin placement by a low resolution indexed color gif image we will create in Photoshop. The .gif will use a limited palette restricted only to those colors in which the plastic push pins come.
This tutorial assumes you have some access to and basic understanding of Adobe Photoshop.
Step 1: Select a Photograph
I'm using a photo I took of a woman who sells her hats and headdresses on the internet.
Step 2: Decide the Final Dimensions of Your Work in Inches
Step 3: Convert the Photo to a .gif of Limited Colors
Step 4: Count Pixels of Each Color So You Can Buy Pins Accordingly
Using your magic wand tool set to non-contiguous regions, tolerance = 0, visit your tiny 90 x 120 .gif and select each color in turn.
Record the count reported by the histogram for each color, That's ho wmany push pins of that color you'll need.
Step 5: Enlarge Your Guide Enough to See It. Maybe Even Go 1:1
1. Convert your indexed color .gif back into a simple rgb image
2. Set image size to 1600% (my usual choice) and use the Nearest Neighbor (preserve hard edges) option in lieu of the usual Bicubic choice. In our 90 x 120 example a 1600% blowup will produce an image that is 1440 x 1920.
You can stop there, but if you want a giant 1:1 guide (and you have access to a large format printer or know how to use a program like Rasterbator to print the artwork in smaller pieces, then...
3. Leave your artwork at its original enlarged resolution (in our case we are now at 1440 x 1920 after a 1600% blowup of our tiny 90 x 120 .gif) but set your width to 45" (as we had originally decided back in step 2), set your height to 60", and set your resolution to 32 pixels/inch, which is 1440 / 45, or 1920 / 60 (both equal 32 pixels/inch)