Age of Empires IV: Anniversary Edition

While on main menu, check down left of the screen, next to a + symbol, is your tag name.
Press on your name and you will view your personal info level games played etc.
Now on the right on the screen you will see a banner and below a bell.
Click on the pencil under the banner.
While in the screen with all the locked banners just click on the already owned banner select some different colors or shapes and the achievement will unlock !

For Xbox, not PC:

On the Main Menu, push in the left thumbstick, then go down to Coat of Arms, then just press on a Sigil & the achievement will pop.