You can create custom banners in creative.
However, on console, how I play, you have to toggle on classic crafting option. EG, instead of just making a pick axe, or a shovel, or a banner or whatever, you have to make it where you have to put the items in the right spot. You know classic crafting.
So turn on classic crafting and then follow these steps to make the patterns you want on your banners. You can stay in creative, and after you are done creating the banner you can go back to regular crafting.
You have to turn on classic crafting in the options. Once you got your pattern sorted with the colors you want then just keep one banner and copy it from hence forth. Then switch back to the standard crafting option as some people struggle with classic crafting.