========================================
Main class file;

    public void load(FMLInitializationEvent event) {
        FMLInterModComms.sendMessage("Waila", "register", "<callback method you want to use to register with Waila (package.ProviderDemo.callbackRegister in this example)>");
    }

========================================
Another class file;

public class ProviderDemo implements IWailaDataProvider {
    @Override
    public ItemStack getWailaStack(IWailaDataAccessor accessor, IWailaConfigHandler config) {
        return ((TILEENTITYILLUSIONBLOCK)accessor.getTileEntity()).getFakeItemStack();
    }

    @Override
    public List<String> getWailaHead(ItemStack itemStack, List<String> currenttip, IWailaDataAccessor accessor, IWailaConfigHandler config) {
        return currenttip;
    }

    @Override
    public List<String> getWailaBody(ItemStack itemStack, List<String> currenttip, IWailaDataAccessor accessor, IWailaConfigHandler config) {
        return currenttip;
    }

    public static void callbackRegister(IWailaRegistrar registrar){
        registrar.registerStackProvider(new ProviderDemo(), SOMEILLUSIONBLOCKHERE);
        registrar.registerStackProvider(new ProviderDemo(), SOMEILLUSIONBLOCKHERE);
    }

}