Bring the loom out of your inventory and add a blue banner and white dye in it. Then you will need to select the design to have a new blue banner.
Add this blue banner again in the loom with white and select the design of the banner as shown in the image below.
The next thing you will have to do is to take this banner again into the loom with a blue dye to have the final design. Bring this banner into your inventory. Add this banner on iron bars to complete your parking sign.