Here is a complete guide for xs++ 3.1.

XS++ 3.1 Guide-Flashing,Customizing,Recovering

Follow step by step.

And I'm also getting the "Coniguration error. Please contact your service provider or service center." message at startup. Will installing the custom pack fix that or did I mess up when flashing?
Yes, you don`t have right custom pack installed.

Nice day