Guide to Opening Kindle eBooks (.mobi) Not Purchased on Amazon

As a writer and reader you might find yourself reading or sharing ebooks that have been saved in Amazon Kindle's .mobi format. I do this all the time when reading a book for a fellow author in the promise of leaving a review on Amazon or Goodreads. But maybe you've gotten that .mobi file emailed to you and after you open your Android Kindle App you suddenly have this thought, "How the heck do I open this thing in Kindle?"