There is no way for an end user to install Windows Phone on a non-WP device.1
The possibility you're referring to, of installing Windows Phone on hardware originally designed for Android, is meant for OEMs, to lower the threshold for entering the Windows Phone market (no need to design a handset from scratch). To achieve this, Microsoft added to Windows Phone 8.1 support for things like dual SIM cards, soft keys and new Snapdragon processors, but also removed certain requirements like a physical shutter button.
For more information, see the following links:
Update: This may change with Windows 10. Apparently Microsoft are planning to release a Windows 10 ROM for Android phones by Xiaomi, that will replace the stock Android install. It remains to be seen whether this is just an experimental thing to get Xiaomi on board with Windows, or whether it'll be expanded to include other manufacturers as well. More information at TechCrunch and Windows Central.
1 Actually, some enterprising modders have managed to run WP8 on the venerable HTC HD2, at least in some proof-on-concept form, but that should be taken as the exception, not the norm. The HD2 is famous for being exceptionally mod-friendly, enabling it to run virtually every mobile OS in recent history, though with varying levels of actual usability, and obviously at the expense of losing the warranty.