In general it should be bright, bold, and simple. A device consists of a field of ordinaries covered with charges in different tinctures. The field is the background of the device. Ordinaries are basic geometric shapes such as waves and stripes. Charges are objects and animals on top of the field such as lions, wreaths and swords.
Tinctures are the colors and patterns on charges and ordinaries.
Try to use at most 2 or 3 tinctures in your design. Try to use at most 3 different objects or charges in your design. Overall, keep the design simple, balanced and eye catching. Here is a Blank Shield to use as a foundation for making your own shield.