Any critique, even a simple one, starts with a thorough analysis of the text ("text" in this case also refers to movies, paintings, car detailing or whatever other creative endeavor is the focus). The critic should ask himself what the apparent intention of the text is, the points it presumes to make and the way it goes about organizing itself. Look at the evidence the creator has assembled in support of his argument, the images he chooses to invoke, the effect the images have, and whether or not he seems to grasp the implications of his work. You might also ask who the intended audience is and whether the text meets its expectations. (A big summer movie aimed at teenage boys should be critiqued under different standards than a sensitive adaptation of an 18th-century English novel.) For a simple critique, stick to the broad strokes. A general overview highlighting the most important aspects of the piece keeps things contained and prevents you from becoming bogged down in minutiae.
Start the critique by summarizing what you've gleaned about the text: what you think it sets out to prove (or display) and whether or not it achieves those goals. Describe what you think works about the text, then talk about perceived failings or areas that could use improvement. Keep your comments relegated to a single sentence or two--just enough to make your point--while adding a few sentences of evidence in support of each specific critique. Avoid all-encompassing statements or blanket judgments--they serve no purpose. Instead, look for more open-ended statements that invite the reader to approach the text from a new perspective. Aim for constructive comments rather than simply tearing things down. A discussion on ways to improve a given piece is far more fruitful than one that simply savages it. Limit the critique to a few paragraphs and perhaps 500 words. That keeps your ideas to the point and retains a certain simplicity, which lets your readers grasp the concepts easily. Finally, be prepared to entertain criticism of your own from people who disagree with you. Accept those comments with the same kind of open-mindedness and try to build a meaningful exchange from it.