Start early. Don't wait until the last minute to put it together. There are several practical reasons for this. First, it allows you to rewrite your proposal several times, and increases the possibility you can have a colleague read it for you and give feedback. Second, it gives you extra time to track down supplemental information such as official transcripts, financial aid records from your school or signatures from deans or school officials. If you need a document at the last minute but you have to order it through the mail, you could miss the deadline.
Review the priorities and mission statement of the funder. As you write, that you keep them in mind so that your proposal reflects alignment with the organization awarding the money. If your research at school or studies are in the same field of interest as the foundation, emphasize this. Do the same for personal qualities or interests that are relevant.
Write clearly and succinctly. Use concise sentence in the active voice. Describe your academic achievements and aspirations with confidence, but without overdoing it; let them speak for themselves.
Assemble all necessary supplemental materials such as documents from your school or financial statements in the manner requested.
Proofread the proposal, and rewrite it as necessary. It is best to put it away for a couple of days and them come back to it. Have a friend read it for you as well and give you feedback. Consult with teachers, if they are available, to make sure the proposal is as solid as you can make it.