Hi, there is already a thread for this purpose that may resolve any issues you are having. Additionally, see this resource for common reasons your custom skin was not accepted.
To submit a custom skin, you can follow this link and scroll down to upload your skin, or go to Pets -> Submit a Custom Skin.
To submit a custom apparel, you can follow this link or go to Pets -> Submit a Custom Apparel.
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For beginners, the easiest way to make a new skin (in my own opinion) is to open a desktop based artprogram such as GIMP or Krita (both free), then import the Beastkeeper provided templated. (You can find this on the skin submission page, under Official PSD bases.) PSD is a format used by photoshop, that is so popular that almost any art program will easily open and save in this format.
When your template is open, you will see different layers. These layers all have transparency locked, which means you cannot color outside of the lines. (Transparent pixels cannot be altered, only colored, and since the image already has color, you will color on the color.) Select a color and use the paintbrush tool to color the 'Base' layer. :) Picture Link
When done, save your image as a .png (make sure not to have a background!!) then upload.
Of course, you can get far more advanced than that- by say, merging all of the layers together and altering it using a brush set to 'color' or by coloring the base, changing the color of the highlights/shading layer, and adding new layers. Texture brushes such as appaloosa spots (not clip art, but textures) can help you be more creative with less time spent trying to get things just right.