To turn on Multi Window, simply head into your phone's settings and find "Multi Window" under the "Sound and Display" subsection. Once turned on, you'll notice a small curved icon with an arrow in it pop up on the left edge of your screen — that's the button you use to pull out the Multi Window drawer and manage your windowed apps. You can press and hold the "back" button on your phone to hide the button, and long press it again to launch Multi Window completely.
