New guy needs basic help with Calibre

Hi Folks,

Please excuse my basic lack knowledge about Calibre but I could use some help getting started. I have been downloading audio books from the Phoenix library for years and life has been good but now my wife has her Kindle Paper White that I need help with. What I am trying to do is download epub books and convert them to mobi and put them on her kindle thru amazon.

What I have done so far is.... downloaded the epub book using overdrive on my windows 10 pc onto ADE. My question is... How do I use Calibre to grab the book from ADE so Calibre can do its thing to change the file format to MOBI?

Thanks,
John