You will need a 20 x 30 paper/cardboard, an exacto knife, a ruler, and a pencil.
Step 1: Step 1
Start from the top of the 20 by 30 and draw a line 30cm, then draw 11cm going down from the ends of the 30cm line. Place a dot in the middle (15cm) anywhere (however long you wanna make the shield). Draw a curve going down the wherever you placed the dot and cut it out.
Step 2: Step 2
Once you cut it out, draw anything you want to make it look like a shield.