Be sure your student has a solid understanding of what a pronoun is and what an indefinite pronoun is. This will make their discovery of negative pronouns much more smooth. Be sure they can identify pronouns and indefinite pronouns in sentences, and explain that a negative pronoun is a type of indefinite pronoun, meaning that it refers to an unknown or undetermined person, place or thing. (See Reference 1)
Try using worksheets to test your students' comprehension of negative pronouns. (See Reference 2) Worksheets can be found easily online or in teaching books, and contain several exercises that will strengthen your students' ability to identify and use negative pronouns. A common worksheet activity is to give the student a passage or list of sentences with several negative pronouns included that the student has to circle.
Besides being able to identify negative pronouns, students must also be able to create sentences with them. Have your students write five to ten sentences with negative pronouns that they can read aloud to the class. Have their classmates judge whether or not the student used negative pronouns correctly in each sentence, then have the classmates take turns identifying the negative pronouns.
It is also important that your students learn to use negative pronouns in everyday speech. Match the students into pairs and have them ask each other questions, with the only rule that the answer must include a negative pronoun. Observe the class as they take turns asking and answering to be sure that they are using negative pronouns correctly.