package untouchedwagons.minecraft.custom_mobs.renders;

import net.minecraft.client.model.ModelBiped;
import net.minecraft.client.renderer.entity.RenderLiving;
import net.minecraft.entity.Entity;
import net.minecraft.util.ResourceLocation;

public class PlayerRender extends RenderLiving {
    private String player_name;

    public PlayerRender(String player_name)
    {
        super(new ModelBiped(), 0.5F);

        this.player_name = player_name;
    }

    @Override
    protected ResourceLocation getEntityTexture(Entity entity) {
        return new ResourceLocation("custom-mobs", "skins/" + this.player_name + ".png");
    }
}

 

This works fine if the skin is from the previous system but the entities' textures are all messed up if the skin is using the new format.

Make a new model.

Oh god I don't have the slightest clue what's involved with that.

Look at ModelBiped and the other Model classes. You can use Techne to model it and generate the code. The main problem would be making all of the texture offsets match the new skin layout.