It depends entirely on the design of the motherboard. Consult your motherboard documentation or technical support options to see what your choices are. If you have the socket, consult your manufacturer for the exact specifications of the chip required. Then purchase and install it, making sure to follow the manufacturer's instructions, and you should be all set. You may need to change a jumper on the motherboard.
Cost is probably only a few dollars.
Some motherboards accept a COASt module to expand the size of the cache that contains an extra tag RAM chip on it as well. This situation is what sometimes makes people think that it is the adding of extra cache that enables more memory to be cached. In fact, it is the extra tag RAM chip on the module, not the cache chips themselves. Make sure you find out exactly what you are buying, and if it will work with your board. If you don't have a socket and you can't add tag RAM via a cache module, you are basically out of luck.