(Edit: added pictures)
First go to this website:
http://www.newgrounds.com/portal/view/571250From there, you can make a custom or easy skin. The easy skin is recommended for beginners as you just click on random stuff and it will be put onto the skin. The custom skin maker is for people who want to really work hard on the skin. It allows you to alter every single pixel in the skin. You can also do a mixture of premade and custom. Once you are done, copy the code your are given, then go to minecraft.com, then click on profile (or maybe it's account i forget XD), then upload your skin. IMPORTANT! YOU MUST HAVE A BETA ACCOUNT SIGNED INTO TO MAKE A SKIN! Enjoy those skins! I'll be looking in classic and on the Beta server!
-LeChosenOne
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~Ask not what your server can do for you. Ask what you can do for your server~