Summarize:

You can hot glue or hand-sew this on. Match the width of the pompom to the width of the white trim. For example, if your trim was 1 inch (2.5cm) wide, then your pompom should be about 1 inch (2.5cm) wide too. Use a store-bought pompom or make your own using yarn.
Add a big, white pompom to the top of the hat.