When I last did the VAT refund at CDG, I did indeed go through the procedure before checking in. In that instance, I did not have to show the stuff that I'd claimed; on another occasion, I did have to show it. Obviously, it's best to put it in a readily-accessible place.

At CDG, look for signs pointing to the Detaxe office. There's also a mail drop adjacent to each detaxe office at CDG, so it's easy to mail back the stamped form.

You might want to allow a bit of extra time. Sometimes there's no one in line at the detaxe, other times you'll be behind someone with 77 boxes of stuff that they're taking back home with them.