Provide evidence or examples. This is the key to a successful letter for this course. You must cite your research. You must also show that you can use the information you have collected to construct an effective argument for action. You might say that it requires putting information in action.
Evidence is always logically persuasive.
Activate your reader's imagination. Invite your reader to engage with your idea, perhaps by using rhetorical questions. Get your reader to participate in your text. Now that you have shown your reader that you see things from his or her perspective, start to turn the tables a bit. Get the reader to identify with your reasons for being involved in this project, and present your reasons in the best light possible.