The battery holder needs to fit inside the battery compartment, so remove some of the ribs molded in the plastic to hold the battery in the compartment and the battery door.

Use a hobby knife or side cutters to remove the ribs outlined in red.

Do not cut yourself. If you are not good with a hobby knife, there are other methods of removing the ribs such as a small rotary tool with a sanding drum, flush cutters, or even heat (again, be careful).