You have to produce the goods (and the claim form the retailer supplied when you bought them) at the airport/seaport where you finally leave the EU. You will either be paid on the spot or sent a cheque later.
Be aware that if you do this through a clearing agency (the retailer's form will indicate whether this is the case) you are likely to lose a substantial part of the VAT refund in commission (UK experience). If it is a generic form and the retailer undertakes to refund you he may take a fee before refunding and you need to make certain you can find out the position and chase matters up. Much depends on the retailer's attitude.