If you are working with a community, you suddenly realize that it is facing some problem or the other. It could be extreme poverty, poor healthcare, lack of government services, malnutrition, inadequate education and so on. You try to go deeper into the problem to understand why the problem exists. For example, if the community is facing health problems, you realize that it is may be because of lack of health centers or the community members are not aware about basic health and hygienic care. Then you understand that the cause of the problem is that absence of healthcare facilities or poor knowledge of the community on health and hygienic care. When you speak to the community, you come to know more and more about the causes of the problem they face. So now you have a clear idea what you need to address. It could be setting up healthcare centers where your community resides or it could be just providing healthcare information to them.
When you are aware of the cause of the problem, you can start writing the proposal. You can develop a background to the problem by referring to its cause and how it can be addressed by your intervention.