In the following steps you will be cutting the adhesive under the battery, and prying it off of the rear case.
The adhesive is laid out as shown in this image: two strips along the long edges, and two shorter strips perpendicular in the center.
To avoid damaging the battery, use plastic tools, heat, and patience. Do not puncture, bend, or overheat the battery.
Do not pry near the logic board, you could damage your iPad.