Lay the device on a flat surface with the screen facing downward. Use one hand to hold the device down in place. With your free hand use one of your fingers and stick it in the gap (as highlighted in the photo)and pull it up towards you.
The gap is located on the left side of the device in the upper left hand corner and the on the right side of the device in the upper right hand corner.
If the battery is not coming out with this technique, use the plastic opening tool to stick it in the gap and pry it up towards you.
The battery will pop out.