Read the research paper that you are critiquing and make notes in the margin. Highlight areas where you think there was a methodological weakness, and areas where you think that the conclusions were drawn incorrectlly.
Take your notes and use them to make bullet points on a new sheet of paper. Your critiques can be divided into two main sections, the methodology and the conclusion. Also note which areas of the research was strong.
Write your critique, beginning by outlining the original research. This should take around one page. Include the important details of the research, as well as the rationale behind it.
Write your criticism, citing from the research where necessary. Your criticism should be roughly two pages. Attempt to explain why the original research took the methodology it did. Often methodologies have to balance available resources and academic value. Be fair in your criticism. At no point should you venture into personal criticisms of the author, and avoid being unduly insulting about the research.
Finally, include a recommendations section, where you make suggestions on how to improve the research. This can be as extensive as you like, and should address all of the areas you have criticised. This should take around a page, although it can be longer if necessary.
Include the full title of the research at the beginning of the critique to help the reader locate the research project you are talking about.