To apply a Sanpella promo code, follow these steps: 1. Select the items for purchase on the Sanpella website. 2. Proceed to the checkout page after choosing the items. 3. On the checkout page, there will be a field to enter the promo code. Paste the code in this field. 4. Click on "Apply". The discount from the promo code will be deducted from the total order cost. 5. Complete the checkout process to enjoy the discounted purchase.