How do I properly return an item to an eBay seller for a refund or partial refund?

[dana]

I tried Googling this first, I promise. I just want to do everything correctly because I'm not eBay savvy.

-I ordered an item. It's the wrong item. I open a case with PayPal resolution center, wanting a full refund.
-The seller responds and asks me to return the item, then she'll send the correct item, and issue a partial refund to cover the shipping cost.
-I agree. I'm not sure if there's a certain process I have to put this package through.

So do I just send it right back (with a tracking number) and that'll be that? Do I have to involve PayPal somehow, since I filed a claim through PayPal?
Oh, and when she said she'll pay for return shipping. Would I just tell her how much shipping cost? I'm gonna get a paper receipt from the post office. How exactly should I handle that?
I feel like I should already know this stuff. =/