Turn on your Firestick and connect it to Wi-Fi
Go to the Home screen
Select the Search icon
Type the app name
Choose the app from the results
Select Get or Download
Wait for the app to install
Open the app from the Apps & Channels section
If needed, sign in to the app
If the app is not in the Amazon Appstore, enable Apps from Unknown Sources in Settings
Use the Downloader app or install the app through a compatible APK source
Open the installed app from Your Apps & Channels