For this tutorial, I will be using Photoshop CS5 (CS3 should work just fine.)
Step 1: For this tutorial, any sepia colored photograph or image will work. Sepia colors or images are dark browns and tans to give an "old" feel to images. They are not in grayscale nor in black and white. They're RGB (sometimes CYMK) but only use brown neutrals as the colors. Oh, and by a "cool retro edge," I'm referring to cool colors... not how awesome this effect looks.
Step 2: I will be using this image from sweetandtalented.com, but as stated before, any sepia toned image will work just as well. I am going to crop the border of this Ben Affleck image because it gives it a tacky Instagram effect that gives me the creeps.
Step 3: Now we can get down to the real editing. First we will create three layers by going to layer - new - layer or by pressing shift+ctrl+n three times. Fill the layers in the order and with the settings and as the image shown to the left (or whatever colors you'd like to use and work best for your image.) I will be using colors: 222f37, 185412, and b8d12b in order from bottom to top. After that, duplicate your original layer (the image you're working with) by going to layer - duplicate layer. Drag this duplicate all the way to the top of your layers window. Set this layer to color burn 50% or whatever works best with your image.
Step 4: Step 3 just set undertones to the image. This will give us some color to work with. However, the real magic comes from altering the levels (intensity of specific colors) to the image. By starting this, go up to layer - new adjustment layer - levels. Photoshop will automatically select "RGB" to alter first which alters all colors at once. Since I want to do them indidually, I will start with blue and then go to greens, reds, and then finally go to RGB. All the settings are shown below:
Step 7: The levels layer should be above all other layers and kept at normal 100%. That's all there is to it. Be sure to save your new creation and credit the original image and resources you've used.