That, in fact, is quite simple. Go to your Program Files and click on Mount&Blade Warband. Within this folder, you should be able to see another folder called "Textures". Go there, and look for files called "banners_a.dds", "banners_b.dds", "banners_c.dds", "banners_d.dds", "banners_e.dds", "banners_f.dds" and "banners_g.dds", select them and press Ctrl + C to copy it. Make sure you don't press Ctrl + X and remove it from the original game. Now go back to the Mount&Blade Warband folder you have been in before, and select the folder "Modules". Here, go into the folder of your module, and again choose the "Textures" folder, where you press Ctrl + V to paste the .DDS files you copied before. You can open .DDS files with Paint.Net! That is important, because that will be your next step. Open the first of the files, and copy paste your previously made banner onto it. Make sure it is in the right position, otherwise it will not be correctly displayed ingame. The banner will take the position of the one you overwrite. You can overwrite every single banner in the files.
It is easy to test whether you did it correctly. For that you simply start the game (remember to restart it after every change you made, and to select the correct module), go to Multiplayer, and edit your character. You automatically have to choose a banner, and you will be able to see where you added your banner and whether it is displayed correctly.
