If you try to install non-market Android apps on a factory-set Kindle Fire you’ll run into some resistance. But, like other Android devices, the Kindle Fire has an option in its settings menu that will let you enable installation of apps from unknown sources. This is known, as sideloading.
Tap the top-right settings area of the notification bar on the Kindle Fire. Next, tap the (+) More button on the drop-down menu.
Tap the Device category in the Settings menu.
Scroll down the screen find Allow Installation of Applications From Unknown Sources. Tap the ON button.
Done. Sideloading is now enabled. Now you can install any Android app on your Kindle Fire. You’ll also need file explorer to open up your apk’s.