Can't build a custom shield in Minecraft on xbox

I'm trouble making a custom shield in Minecraft on my xbox, I made one before awhile back, but I added some other stuff to my banner and I don't know why it's not working anymore. I tried adding less/nothing on my banner and I still can't attach the banner to my shield. I don't know if I have to have a certain XP level to make a custom shield or I just can't do it at all. If you can tell what I need to do to fix this problem I would I appreciate it.

Thank you