How can I add a cape to my chest model?

[vital idol]

I just finished my entire model and I only need the cape but I don't know how to add it, I tried some tutorials or reading old messages but the entire model became bug, plss can someone help me?

[vital idol]

The problem I also have when I manage to put the cape is the weights painting but no matter how much I paint those areas it doesn't seem to fix itself.

here is the bug on game