It is highly advisable to create an outline before you start writing your reaction paper. Doing so will help you determine beforehand how many "reactions" you have. It also affords you the opportunities to search the justifications you need and to list the supporting theories to them.
In a paragraph, start your reaction paper with your introduction that includes a brief summary of the material you are tasked to respond to. Devote at least three sentences for this purpose. Remember to include the title and author of the work you have summarized. At the end, write your thesis statement.
The second paragraph should contain your first "reaction." Write it concisely, preferably in one short sentence. Follow this with your reason for having that perception. In both cases, always refer to the work your are responding to. For instance, after stating that "I think the first chapter of the novel is dragging" continue with another sentence such as "After the first five pages, the protagonist does nothing exciting or provocative." Further substantiate your reason by adding a theoretical perspective that will explain your observation.
Apply the previous method with the succeeding paragraphs for your other reactions. As much as possible, use personal indicators such as "I believe," "I think," "I suppose," "In my opinion," and others.
End with your conclusion. Conclude with a restatement of your thesis and