This is very crude but it should at least help you get going. You need to put this in a custom ArmorItem class and have your armor items be of your custom ArmorItem:
@OnlyIn(Dist.CLIENT)
@Override
public BipedModel getArmorModel(LivingEntity entity, ItemStack stack, EquipmentSlotType slot, BipedModel _default)
{
YOUR_ARMOR_MODEL model = new YOUR_ARMOR_MODEL();
return model;
}
@Override
public String getArmorTexture(ItemStack stack, Entity entity, EquipmentSlotType slot, String type)
{
return YOUR_TEXTURE_LOCATION;
}