At airports, validation is mandatory before checking in your baggage (in the departure hall).
Types of validation:
Electronic (France and Spain): directly from the screen of your smartphone, present the barcodes of your tax-free forms under the optical reader of a self-service kiosk (called PABLO in France, DIVA in Spain). Alternatively, present your smartphone to a customs officer who will validate them using an electronic gun or by manually entering the barcode numbers on his terminal.
Electronics (Belgium): go to the customs office and show your passport -> the customs officer will validate your electronic tax-free forms from his terminal (self-service kiosks are not yet in service in Belgium).
Customs stamp: present your paper tax-free forms to an agent at one of the customs offices at the airport, train station or border crossing point.
In some cases, for your hand luggage, you will be obliged (for example at Brussels airport for certain sensitive or high-value items) or you will prefer (shorter queue, personal or expensive items) to have your tax-free forms validated in the airside zone, after passing through the security clearance gates. In this case, you must carry the items with you when you pass security. Also make sure beforehand that there is a customs desk in the transit area.
the validation by stamp is always done by the customs officers of an EU country (for example by French customs officers -and not English officers- at the Eurostar or Eurotunnel terminal, by French customs officers -and not Swiss officers- at the Franco-Swiss border posts).
See also questions: