Choose a subject that you want to write a reaction about. If you're in a class, this will likely mean choosing a piece of text or a world event. If you're writing for a newspaper, this usually means a current event.
Introduce the subject enough that your audience can understand its essential parts and, as follows, your reaction to them. This part takes up about 1/3 of your overall paper.
Develop an argument that is consistent with your reaction. For example, if you agree with statements made in a piece of text, then your argument should seek to detail those points of agreement. If you think that a certain event should have been handled differently, then your argument would call out the shortcomings of the event's handling.
Lay out your argument systematically and with direct reference to the critiqued subject. If you watched a documentary about how chickens are farmed, you might argue that chickens are treated poorly, and a law should be passed to ensure they are raised in more humane environments. You would cite specific parts of the documentary that show how badly chickens are raised, then, with each example, you might present fixes or alternatives that would lead to possibly better outcomes. This takes up almost the entire last 2/3 of your critical reaction.
Conclude your reaction by briefly recapping what you've discussed. Restate your points, and state in a few sentences how you've supported them. Finally, express your ultimate point -- that a law should be established for humane chicken farming, for example -- and instruct how other people might carry on if they agree with you. This might mean writing their state senator, for instance.