What are the client's problems?
If you want your proposal to be the winning one, make sure you spend ample time understanding what your client is facing. In this section, clearly explain your best understanding of the client's problem. Spend extra time on formulating this part, as it may very well decide if the rest of the proposal will be read or not
If your proposal properly answers this question, then you're on the right track to writing a winning proposal. Get into the finer details of how you're going to solve the problem, explain in detall while making sure to keep projections accurate and realistic. This is also a great section to highlight any knowledge that can set you apart from the competition.
How are you going to solve the problems?