Sure, you've been playing lots of Mount & Blade 2: Bannerlord, but can you truly call yourself a lord of banners? If you're just using Bannerlord's standard in-game banner editor, probably not.
Luckily, there's an online tool that you can use to create a custom banner—and amazingly, it spits out a code you can paste right into the game itself. Create your banner and a second later, there it is in the game. You don't need to install a mod or anything.
Start by visiting the banner creation site right here, courtesy of this Reddit member. It takes a little testing to figure out how it all works, but you can add all the standard elements to your banner and actually change their size and color, add multiple shapes and images, rotate them, move them around, and so on. Once you're got a design you're happy with, simply copy the code directly below the banner window with Ctrl-C.
Now launch Bannerlord and load your game. When you're in map view, pressing B will bring up the banner creation window. It seems like it shouldn't be this easy, but it is: just press Ctrl-V to paste in your banner code. There's no field or or anything to paste it into, just paste right into the game when the banner editor is open. It's magic.
And if you want the code for the PC Gamer banner picture above, here it is:
Just copy and paste it! And if you can make a better one (I'm certain you can) please send the code our way.