1) First, from your Google Now homescreen, simply say “OK Google” to bring up the voice search window
2) Next Say say “OK Google Everywhere”. Google will display a search results page, simply click on a link (any link) ...this unlocks the settings you’ll need to adjust in the next section.
3) Next, go to Settings, Language and Keyboard, then select Voice Search.
4) Select “OK Google” detection, and check the boxes next to where you want to be able to search from the Google Search App from (any screen and even from the the lock screen).
5) Google will have you run through a quick training to teach it how you say “OK Google”
Thats it.I am still testing the effect on the battery but it doesn't seem too bad. Having the ability to use "ok Google" from the lock screen is a big time saver.