For this instructable, I inspired myself of all those watches battery changing tutorials videos because they all have a tool in comon : The screwdriver for watches. I thought that it was useless to buy that tool.
All you need is :
-a watch with a back you can unscrew ( like in the picture )
-a pair of scissors
-a wire (a pin)
-not a new battery, you don't know the battery number yet, (they are not all the same)