Watch a movie carefully. If you watch it with others, make sure they are reviewing it for content and not just entertainment, so they will not distract you.
Take notes on the movie while watching it. Note each profanity, how it's used and how often. Note instances of nudity for duration, context and sexuality. Note violence by type, graphicness and context. Pay attention also to the general theme of the movie, asking yourself if it is something only an older audience could appreciate.
Review your notes after the movie has ended, making tallies of the amount of mature content. If you are rating with someone else, discuss and compare notes.
Give a movie that contained very little to no realistic violence, language, sexual content or other mature themes, a "G" rating for General Audiences. If there is mild violence and/or brief mild profanity or non-sexual nudity, give it a "PG" for Parental Guidence Suggested. If it contains more than brief nudity, a few instances of graphic violence or harsher swear words, if it deals with themes children could not understand or should not be exposed to or contains any drug use whatsoever, give it a "PG-13" for Parents Strongly Cautioned.
Rate a film that contains heavy amounts of drug use, violence or sexuality or more than one instance of harsh language an "R" for Restricted, particularly if you would not consider it appropriate for a child of any age. If the movie is excessive in any of these areas beyond what any child should be exposed to under any circumstances, such as learning history, rate it "NC-17" for No One 17 and Under Permitted.