The first thing you need to do to accomplish this task, is to download the latest updated version of Google Search for Android from the Google Play Store. Once that is done, you download the Nexus 5′s GoogleHome APK. The only settings change you will need to make is the one that allows you to install non Google Play apps on your phone.
With the Google Search app and the GoogleHome APK running on your phone, you can select the Google Now launcher by pressing your phone's home button. And while it has been designed for the Nexus 5 only, those who have tested it out say that it runs smoothly on the Samsung Galaxy S4 and the HTC One. If you have the Google Now launcher running smoothly on another Android device, let us know by passing along the information in the comment box below.
source: GoogleSearchAPK, GoogleHomeAPK via RedmondPie