This pack only overrides the default Steve and Alex skins with your custom one.
Replace the steve.png and alex.png files in assets/minecraft/textures/entity with your skin.
You can optionally insert your name in the according space of the pack.mcmeta file and change the icon with the Mine-imator project.
Then place the folder (or .zip file) in the resource packs folder of your Minecraft installation and activate it in game.
Next time you play (especially when you're offline), you will have your skin on instead of the default one!
Note: All players with default skins will wear your skin as well, so just deactivate the pack when you're online!