At the airport or port, go to the customs desk and present your completed tax free form, passport, receipts and purchases to get a stamp on your form.
Where to get your stamp?
Note that you can only claim your tax refund at the last european (air)port from your trip back. Example: if you have a direct flight from amsterdam to the usa, then you need to get your stamp in amsterdam.
Where to get your stamp with layovers?
For example: if you fly from the netherlands to the usa, but have a layover in france, you need to claim your stamp in france, not in the netherlands.
If you have a layover outside the european union, you still need to get the stamp in europe. For example: you have a flight from amsterdam to australia with a layover in china, you need to get your stamp at the amsterdam airport, not in china.