1. Open Miro. (Download Miro here if you don’t have it.)
2. Connect your Kindle Fire with a USB cable to your computer and it will appear in the ‘Connect’ tab in Miro.
3. Drag any videos or music to the Kindle Fire in the Miro sidebar and they will automatically convert to the right size and format and sync.
Note that videos that aren’t purchased from Amazon will be viewable in the ‘Gallery’ app on your Kindle Fire, not in the ‘Video’ tab.
we also put together this simple site explaining how it works: Sync Video and Audio to Kindle Fire