Amended tax return, what to do?

I filed my 2008 tax return with H & R block in January. A few days later I received another W2 form from an old job that I forgot I received a pay check for in the beginning of 2008. Now I have to have the taxes amended but H & R block wants to charge me another $100!! How hard would this be to do myself? What are the penalties if I do mess up? Does the IRS give you notification first, or straightforward penalize you?