So... if I won a prize in a forum game and receive a PM asking what prize I want, all I have to do is reply, correct? Or is there some sort of other thing I need to do?

In your PM, you have to reply with what forum game item you want. (it's an avatar item name)

You get to choose any forum game item from the Items tab of your wardrobe. All forum game items should have this icon (of a small head of a female elf) in the corner of the item.

They should start at approximately page 3 or so of the Helds section, page 1 of the Backs section, and can be possibly found in the other tabs too (like there's maybe a few in Outfits and Wings).

I got to choose a 3 vis themed pet (2vis if majestic). I replied, but I'm not sure if I did it right

Oh, is that your prize? Sorry, I thought you meant an avatar item aha.

You just pick whatever theme within Fall Festival 2017 that you want as a 3vis (or 2vis if majestic), then PM him the theme. o:

That's it? I hope there isn't some sort of secret procedure or something ow0

Nope, it's pretty straightforward. c: Congrats on your prize!

Thanks :D