When you are notified that gifts are ready in your account you can log in and access these gifts from the “Gift Campaigns” tab.
To the far right of each draft gift (you may have to scroll over to see it), you’ll click the “Edit” button that looks like a pencil in a square: 
You can personalize the message anytime during the draft period up to 1 hour before gifts deliver. (Delivery queue date and time displayed next to the gift in your account.)
Don't see Draft Gifts under "My Gift Campaigns?" This likely means that the newest gifts aren't quite ready yet. You'll receive a notification when gifts are ready in your account and you can edit them at that time.