I apply the ribbon to a freshly coated buttercream cake so it will stick the best. I have never had luck with the wax paper technique, so I spray the whole ribbon with Pam or pan spray to get an even coat of grease on the ribbon. That way, the ribbon doesn't get splotchy. Sometimes it will change the color of a pale ribbon to make it a little darker. If you explain that to the bride, there shouldn't be a problem. If they buy the ribbon and get what I specified, I don't charge additional to place it on the cake.
Subtract the ribbon width from the height of the tier and divide by 2 to get the distance from the top or the bottom of the tier spot to place the ribbon.
Use a hemming ruler or other ruler to help you mark little spots in the cake you can follow to apply the ribbon in a straight line. That is much harder to explain than to show!