Start by loosening the 16 Phillips #00-headed screws in the midframe, 7 on the top half, the remaining 9 on the lower half.

Now try to get a plastic pick in between the colored midframe and the black plastic frame around the display. A opening tool might help you. I found it the easiest to start in the top right corner (where no camera bump is), but this might vary between devices.

Then work yourself around the device to unsnap the two frames from each other. Be careful around the sim card tray, the plastic frame is quite thin there. Also try not to go to far into the device to prevent damaging the internal components, especially the cameras.

Unplug the battery flex cable.