Need to detangle a bunch of things here.

Firstly getting your reclaim form stamped: (n.b. for the benefit of anyone new reading this, it is not simply your till receipt, you should have a dedicated VAT reclaim form provided by the store, and this is not the same as duplicate receipt highlighting the VAT).

To get your form stamped, you need to show the form and the goods to a customs officer.

If the items on the form will be carried in your hold bags: show the items and form to the customs officer BEFORE you check the bag in the last time inside the European VAT area.
So if you are flying BCN-CDG-IAH and your baggage will be through checked, then show it in BCN.
If they are totally different airlines and won't through check, then show it in CDG.

If the items will be carried in your hand/cabin bags: Show the items and the form to a customs officer at the last airport you pass through that is inside the European VAT area - for example CDG if you were flying BCN-CDG-IAG. Hand items are generally shown during the course of passing through security to enter the departure lounge, or just after. Check local airport procedures, just in case you need notify them before going through that you want to do this.

NOTE: Customs officers will generally not stamp forms if they can not see items, nor can they partly stamp a form. As a result, if you are buying items in a single purchase that will be exported by a mixture of hold and cabin bags, then you should get them put onto separate reclaim forms. (e.g. a bulky safe item vs small delicate ones you might want to carry in the cabin). Also the situation if different people leaving on different days will be carrying the items.
Even though this will double up the reclaim fees.

If you have an item that you will not be exporting in the end, or are not able to show the customs officer because you have packed it separately, then most forms do allow you to put a line through that item. Check the provisions of the form, and discuss with the customs officer if need be.

Getting your refund - i.e. what to do with your stamped reclaim form:

The methods by which you can get your refund will vary depending upon:
- What methods your refund form permits (shops partner with different refund companies, some shops do it inhouse).
- What private refund facilities exist at the airport, especially if you have already reached the departure lounge.


Some refund forms will allow you to choose between:
- Refund onto credit card that made the purchase
- Cheque posted to you. (with options for it to be in your local currency)
- Getting paid cash at the airport from a 3rd party provider (if such facility exists at that airport and is open, and partners with your refund company.)

Your reclaim form will generally indicate which options are open to you.