1. Every time you sell the product
Tell your customer:
"I think you should get our regulars offer".
Followed by: "You'll get a free X after Y purchases of Z"
Followed by: "You just download stamprapp from App Store and Google Play and you will get the card and first stamp, even without signup."
2. Print the offer and put at the cashier
This will be provided as a pdf for your specific offer.
3. Print the offer at the end of each receipt
This requires a cashier system that support configuring the text on the receipt.
4. Share the offer in social media
This is how a restaurant in Sweden decided to do it on their Facebook page.