Create an outline
Explanation:
Creating an outline is one of the process that helps outline or highlight the important information by following a proper step by step process.
The main purpose of creating an outline is that it make the content or any type of important more effective for understanding some main points and it also preventing fro any type of stuck.
According to the given question, the Jonas next step is to create an outline about the decision made by principle regarding the uniform. In the letter, he outlining all the problems and explain briefly that he is disagree with the decision.
Therefore, Create an outline is the correct answer.