Custom Backpack Skin

[Kyoobot]

I am trying to make a character that has a cape. I tried to put it into the hair slot, but the results looked terrible, as the hair is meant to be to the sides of the character, not behind.

So, I'm thinking reskinning a backpack might be a better choice to help with the issue, I chose a backpack considering you'll lose item slots if it was anything else. I have been trying to find tutorials that can help me get started, but none have helped me so far.

What i need help with is how to put a custom item into a characters mod, as in just downloading the character will give you the item.

I've already gotten the character themselves in, so I have the file set up for that.

Thank you very much in advance for any help!