How to use the Hatsune miku skin I found but naked skin

Download the PNG above — it is a 128×128 HD file built for the Alex (slim) model. At four times the usual detail it needs OptiFine, Figura, CustomSkinLoader or a server that handles its own skins; minecraft.net only accepts 64×64. To change it first, or to resize it down to 64×64, open it in the 3D editor and paint straight onto the character.