Um, no you didn't. But whatever, first go to deviantart (or just google) and download a few sets of nice dirt brushes. (I'm assuming that you use photoshop or something similar with layers and brushes). After you have the brushes, I'd recommend to make a new file for a dirt kit. Use the brushes to apply dirt, and when you are happy with the result, copy&paste it on a skin you want. Save the dirt document as a transparent .png for later use.
Making dirt kits isn't that hard, but it takes a lot of patience aligning/blending the dirt over the corners. Also, use multile brushes to avoid patterns.