Film Processing is using a developing agent to reveal the image and a fixing agent to ensure it does not fade away. Traditionally developers have been fancy chemicals from photographic suppliers. Either in liquid or powder form. However, a few years ago someone noticed that coffee, and other natural products contained the main developing chemical. Mixing coffee with washing soda with a bit of vitamin C produces a very useful developer. The general term is Caffenol. Coffee is the cheapest instant available, washing soda can usually be bought at any store while pure vitamin C is available from specialists. Some people though crush the tablets available. While there are a formulae guides it is best to experiment and find your own preferences.
Below links to the processing pages