APPS2FIRE - is an android app that allows you to install any app to your Firestick/Fire TV without having to physically connect usb cables, flashdrives, ect to it. The transfer is done over wireless network from any other Android device (phone, tablet, ect)
HOW TO:
1) On your Firestick/Fire TV go to Settings > Developer Options and enable both: USB Debugging and Allow Apps from Unknown Sources
2) In your Firestick/Fire TV settings, go to the network section and take note of your IP Address
3) Install Apps2Fire on your Android smartphone/tablet (search for it in Google Play Store, it's free)
4) Open Apps2Fire and in the settings section, enter the IP Address of your Firestick/Fire TV
5) Use Apps2Fire to begin installing apps to your Firestick/TV. You can either install apps that are already installed on your phone or even backed up .apk files saved on the device.
*ALMOST ALL ANDROID APPS SHOULD WORK ON FIRESTICK/TV SINCE IT RUNS ON ANDROID OPERATING SYSTEM. HOWEVER, APPS THAT REQUIRE "GOOGLE PLAY SERVICES" MAY NOT BE COMPATIBLE.
I think this is a pretty straightforward process, but should you have any questions, feel free to post them here and I'll do my best to answer.