The process takes a few classes. In this first stage, the students learn that melody is a line that rises and falls and almost always makes its way back to the home note. They create a line on a 4 bar strip of music and then "plot" notes along it, using what they've learned about rhythm patterns.
Homework -- 4th grade teachers, I'll need your help with this. Everybody needs to come to class next time with their 4 bar strip of music and a drawing based on the line. What kind of rhythm patterns should you use? Check your rhythm sampler for ideas. What should you drawing be of? Anything you want, as long as it incorporates your line.