Sideloading Kindle with access to a computer (easiest):
Using a mini USB to USB cord, connect the Kindle to a computer
Download the ebook file on your computer
Using a file explorer program, open up the kindle (will show as a temporary drive). also open the folder on the computer with the file
Drag the ebook file from the computer to the Kindle Books directory
Follow your computer’s procedure for disconnecting peripheral devices. The book will now be available in the Kindle “Books” tab
Downloading directly from Kindle Fire:
Download the ebook file to the Kindle using either the browser or via email.
Download and install a file explorer program, such as ES File Explorer from Amazon APP market (it’s free)
Using the file explorer find the book file (it will be in the main Kindle directory) and copy it (press and hold the icon until the submenu pops up)
Go to the “Books” folder
Paste the file (with ES Explorer you will need to select “Operations,” then hit paste)
Power Kindle off and back on
Book should be in the Books tab
Here is the link to a support article that explains how to transfer content to a Kindle device via USB: help
Move book from Mobi downloaded on Kindle
Recommend you use a computer then load to Kindle, but if you have already downloaded to your kindle
Go into your amazon account (login)
Go to content and devices
Go to preferences
Go to personal document settings
Make sure your @kindle.com and @normal email address are added to approved personal document email list
Go into your email account on your kindle. Attach your mobi file and send using the second option (one says send and lower says if all else use this) send to kindle account
Go into content and devices
At content, click the button that says books, select docs
You will see your book listed
Under actions click the button with three dots
Click deliver to yourname’s kindle
Your books should show up so you can read them under new items