In your camp menu you can always change your banner (so long as you have one).
If you mean create a custom banner, then there is no in-game way to do that as of yet, but you can use any image editor that can deal with .dds files (The GIMP, Photoshop, etc) and go into your ...\Modules\Floris\Textures\ folder and edit the files with _banner_ in their name.