For this tutorial, I will be using Photoshop CS5 (CS3 should work just fine.)
Step 1: Before we begin, let's establish what this tutorial will affect and what "rustic" means. Just like the name states, rustic colors are often dark browns and warm reds that give off a "rusty" effect. Sometimes these colors are associated with a vintage look as they give off an "antique" connotation. This color combination is popular with lots of other types of designs such as fashion and photography.
Step 2: Now let's begin the steps. Take this image from sweetandtalented.com or another image of similar neutral colors and paste it on to your base layer. This is often called the background layer or layer 0.
Step 3: After that, we will change the layers to make them darker and warmer. Go up to layer - new adjustment layer - levels and change the settings like the image shown below or to whatever looks best with your image.
Take that levels layer and set it to multiply at 50% or whatever percent works best for the image you are using.
Step 4: Now, create another layer by going to layer - new - layer or by hitting shift+ctrl+n on your keyboard. Make sure this layer is above the layer of the levels. Fill it with the color 71674d or any light brown color with a warm green undertone. I set this layer to lighten at 23% but again, you can use whatever percentage looks best.
Step 5: Once that layer is done, we will do the same but with another color and layer effect. Create a new layer (shift+ctrl+n) and fill it with 3e3830 and set it to hard light at 27%. As stated before, feel free to alter the settings to your own liking.
Step 6: That is all the layers we will be doing. However, you might notice that the face of our image has become covered in colors and becomes dull. (If you're using a portrait as your picture, you'll notice this as well.) I took my last layer (from step 5) and using the lasso tool feathered at 20% selected around the face (often the main focus) of the image and deleted the area using the backspace on my keyboard. You don't want to use a feather at anything lower than 15% because it'll look unnatural and choppy. (Like a cut out.)
Step 7: That's all. Be sure to save your image and credit the image and resources you've used.