SCENARIO ONE: Guest has a lot of cartwheel deals on multiple items.
Scan receipt -> Fix a Mistake -> Wrong Payment. This will reopen the transaction, which will allow you to scan Cartwheel. Hit "Total" and have the guest do their payment once again.
*EXCEPTIONS: Pay attention to how the guest paid. If the guest paid partially with a gift card, money will go back to those original gift cards. You may have to go pull them out of the trash from where they checked out if the guest no longer has the gift card. If they paid cash, you may have to pull cash from the original register as well, depending on how the guest pays. The register will prompt you on what to do in this case, so just follow the prompts. If the register doesn't say anything about a "money transfer" then don't worry about it.
SCENARIO TWO: Guest has only a few items on Cartwheel. I'd say 5 or less.
Do the math -> Scan receipt -> Fix a Mistake -> Missed coupon for whatever the amount is.
SCENARIO THREE: Also best used in cases where only a few items where missed, and is probably the most "brand" and "correct" way to handle the issue.
Scan Receipt -> Fix a Mistake -> Wrong Price ->Type in Cartwheel Percentage and hit "Missed Cartwheel."
*OPINION: Even though this is the technically correct way to do it, this isn't my favorite option, because it only lets you do one item at a time. I typically only do this when the guest only had one item for cartwheel.
SCENARIO FOUR: When all else fails, you can always return the items, and resell them, scanning Cartwheel. Typically I do this in "Make it Right" situations where it's been a few days since the transaction, or any other instance where the register isn't doing what I want. If you're at the Service Desk, you know how it is sometimes.