How to download and install Android apps from unknown sources?

Security Setting - Unknown sources

​Allow installation of apps from unknown sources.

Settings -> Security -> Enable "Unknown sources".

Download APK or ZIP

Download the .apk from website.
Some devices deny download .apk file directly, then please download .zip file.

Install APK or ZIP

If you have download a .zip file, please extract it at first, and then run .apk to install the App.