1. Go to https://www.ClassiCube.net
2. Log in to your account if you haven't
3. Click on your username
4. Click "Choose File" below the "Upload a new skin"
(Use SkinSwatch if you don't have a skin, so you can create a skin from the templates. Or you can just search "Minecraft skins" and download one from those websites.)
5. After you've chosen your file, click "Save"
(or click Choose File again if you are not happy with it.)
6. Congratulations, now enjoy your new skin.