Button batteries are small, flat batteries used to power items that don't use a lot of energy. Examples include watches, hearing-aids and key fobs, and they are fitted as backup batteries in cameras and clocks. Inserting replacement button batteries in your electrical items is relatively simple, helped by the clear polarity labelling on the button battery.
Look carefully at the button cell battery compartment, or the reverse side of the battery cover, to check which way to insert your button battery. You will see "+" or "-", meaning positive and negative respectively, marked on the base or side of the battery compartment. If the marking is on the label, it often shows a diagram of the battery indicating which way around it needs to be inserted.
Look at your button cell battery; it has a flat side and a slightly rounded side. The flat side has a large "+" marked on the top.
Insert your button battery into the battery compartment ensuring the "+" side of the battery touches the "+" terminal. Close the battery cover -- either slide it in place or press it so it clicks shut. If it is held in place using a small screw, insert the screw in the screw hole and tighten using a small screwdriver.
Some devices have a slot that the button cell fits into. You will still find the device is labelled to indicate which way you need to insert it into the slot. If you do insert the button battery the wrong way, it won't damage your equipment -- it just won't work.