Simply explain the concept of a coat of arms to your students (not always easy...) and have them draw what interests them most, or what they like the most, on the shield before coming up with a motto for the banner.
You can then have them show their work to the class and give a talk to them about their coat of arms.
If you know what and OHP is and have access to one, you can always get the students to design their coat of arms onto an acetate before showing the glass on a larger scale.
