Be as forceful and dramatic as you can without stooping to purple prose. Second, show how you will solve this problem. Describe your project step by step, taking pains to demonstrate how each step will address the problem and promote a solution. Provide whatever information the reader will need to be persuaded, and no more. Third, describe how you will evaluate whether or not your solution solves the problem.
Most sources require an extensive evaluation section.
Evaluations can be formal or informal, formative or summative, internal or external, depending on the project and the funding source. Make sure that your system of evaluation will, in fact, demonstrate that the need described in the first part was addressed by the solution you propose in the second. Many reviewers look at the budget first, before they read the narrative. They assume that the budget shows what the proposal is all about in just a few pages. The various items in the budget should follow the same order as the activities you propose in the narrative.