1. Setting > Security > Check Unknown sources.
  2. About > Tap Build number 4 times, Congrats you are now a developer!.
  3. Settings > Developer options > Check USB Debugging.
  4. Download an APK.
  5. Connect to PC.
  6. Drag/Put APK on root of folder.
  7. Grab a file explorer (on google play) ex: file explorer HD, ES file Explorer, many other choices.
  8. open file explorer from step 4.
  9. navigate to your APK you want to install.
  10. tap click install on the prompt.
  11. You have now have installed APK.