ugh... no matter now hard i try, i simply can not change fonts on my 350!!

I follow all the directions precisely, but still no luck!! The book seems to be converting, but on the reader the font does not change.
Also, the testing for font changes is such a labourious task with converting, moving the book to device, ejecting device, waiting for it to load etc etc... is there a shorter way of checking whether the font has been changed prior to disconnecting the reader?
Using Windows 7, Firefox and calibre.
Can anyone direct me to some REALLY good guide and discussion of changing fonts?
thanks! I am getting sooo frustrated!!