how to add player skin overlay

You are looking for Layers. However, this is not supported yet in MCreator.

Simply,

You need to create a Layer that extends LayerRenderer. If you need help add my discord (in profile).

You need to add the Layer to all entities using,

    public static void addLayers(PlayerRenderer renderer)
    {
        List<LayerRenderer<AbstractClientPlayerEntity, PlayerModel<AbstractClientPlayerEntity>>> layers = ObfuscationReflectionHelper.getPrivateValue(LivingRenderer.class, renderer, "field_177097_h");
        if(layers != null)
        {
            layers.add(new [YOURLAYER]<>(renderer));
        }
    }

And call the above using,

        Map<String, PlayerRenderer> skinMap = Minecraft.getInstance().getRenderManager().getSkinMap();
        [CLASSCONTAININGMETHOD].addLayers(skinMap.get("default"));
        [CLASSCONTAININGMETHOD].addLayers(skinMap.get("slim"));

Due to this being a popular request, I may do a user tutorial soon. Time will tell.

a tutorial would be great since i still dont have a clue about this

could you please do a tutorial?

yes, could you do a tutorial ? pleassee xd xd

yes please tutorial