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