Once the ebooks are finished converting, you can send them to the Kindle Fire with Calibre’s “send to device” feature by plugging the Kindle Fire into your computer with a USB cable.
It needs to appear exactly like that, with capital letters and brackets.
In the section where it says “Personal Doc tag”, remove the tag and replace it with.
Make sure the Output Format in the upper right is MOBI.
Select the ebook(s) and then click the “Convert books” tab to begin the conversion process.
Import the ebook or ebooks into Calibre that you want to have appear in the Kindle Fire’s ebook library (if you’re new to Calibre, here’s a getting started guide).
How to Get Sideloaded eBooks to Appear in eBook Library