Step 1: On an NFT details page, click on Buy button

Step 2: In checkout popup, input discount code from the Store.

If the Store set his discount code as Public, the code will automatically pre-fill

But if the Store set his discount code as Private or you're unable to use the code, it won't pre-fill

Step 3: Click Proceed to payment

Step 4: Click Confirm in Metamask popup

Step 5: Now, just wait for the blockchain to complete your transaction