Unsigned APKs do only work on rooted and patched devices/environments.
These are not always provided by the mod publisher as they do only work under certain circumstances.
1.) Your device must be rooted.
2.) Your device must be patched to ignore app signatures. This can be done with the help of tools such as Luckypatcher or Xposed.
Once you fill that requirements the process is the same as with signed APKs with the difference that you can overwrite the original game/app with the MOD APK without removing it first.
1.) Download the unsigned MOD APK.
2.) Install the unsigned MOD APK.
For the case the unsigned APK does fail to install: Your device patch is not done correctly!
Google login possible? Yes.
Facebook login possible? Yes. Even with Facebook App installed.
Specific game account login possible (for example: E-Mail, HIVE, Kakao)? Yes.
You are rooted and want to know how to patch your device? Please click here for more information.