When ingame, you can use the command /skin "skinname".  Skin name being the name of the folder in which the skin is contained within.  When you done this, you will have to restart the client for the changes to take place.  When you first download the client, the only available skin is "Default" and "Halloween".  More skins can be added as explained below.

You can find through the following link (http://forum.playerdex.com/Index.php/forum/78-client-art-customization/) that players will upload their own client skin or that they may have their own skin shop if you feel like you need a different skin.  Usually instructions will be provided.

You can either start from the Blank Page or just copy the Default skin folder.  For this guide we're going to do it from copying the Default Skin folder.  This can make the process easier in a way rather than starting from scratch as you can use the original file as a guideline.

It would be recommended to use either GIMP or Photoshop when it comes to skin-making.  We're going to first work on the login screen in Photoshop.

First off once you got the login screen loaded up into Photoshop, you may want to right-click on the Background layer and choose "Layer as background..." and just click Okay on the window that comes up.

So now we can actually start on the designing part.  First off i'm going to start off with working on the login box.  You will find templates of the login box on the forums however for this we want to do our own.  For shadows and glows, I would recommended leaving till nearer to the end as the effect will work different depending on what's before it in terms of layers.

So now we got the layout of the login box finished, we can make a start on the background.