Trying to attach some objects to an already rigged and animated character - they're not 'sticking'

I downloaded a Mixamo animated character and I have some extra bits of items I want to add to the character. I’m not familiar with rigging etc. so I just thought I could parent the item to him and it would effectively attach and move where he walked. That didn’t work so I tried CTRL+J to attach the geometry and still, the object stays right where it is when he walks.

Well i wanted to write a long text but i think these links will help you too…

ah cool, I’ll check that out. Thanks!