The following screenshot guide is for Amazon Firestick/Fire TV users still using the older interface.
Step 1= From your home screen, find the search icon and type Downloader.
Step 2= = Click on the Downloader icon
Step 3= Click Download.
Step 4= Wait for the app to install. Click Open or Launch now.
Step 5= Click Allow.
Step 6= Click OK.
Step 7= You are now on the main screen of Downloader, return back to your device’s home screen.
Step 8= From the home screen of your device, hover over Settings.
Step 9= Scroll to the right and choose My Fire TV.
Step 10= Select Developer Options.
Step 11= Turn on Apps from Unknown Sources.
Important Note: For those using the Firestick Lite, 3rd Gen Fire TV Stick, or 2nd Gen Fire TV Cube, click Install Unkown Apps.
Step 12= Locate and click Downloader. You will notice it says “ON” which will enable unknown sources for side-loading.