Android 5.x (Lollipop)/Android 6.x (Marshmallow) Steps

With the phone powered off, press and hold the Volume Down button and the Power button at the same time until the device turns on.
Press the Volume Down key to highlight Recovery mode.
Press the Power key to restart into Recovery mode.
The screen will display an Android robot with a red exclamation mark.
While holding down the Power key, press and release the Volume Up key, then release Power.
Use the volume keys to scroll to Wipe data/factory reset and press the Power key to select it.
Scroll down to Yes - erase all user data and press the Power key to select it.