I need some advise on creating sand for a beach cake. I plan to use graham crackers mixed with a brown sugar, but after I ice with BC pat the sand mixture on, and write "congratulations" in the sand ....

MY QUESTION IS:
How do I keep the sand from moving back over the letters during transport?

Thank you in advance,
Tammy

Maybe you can take a thick piece of fondant and use your finger to make an impression of "congratulations" (kind of making a ditch). Then cover that with piping gel and then "sand"

You shouldn't have any sand to move into the letters if you blow the loose sand off your cake before transport.
If you want some loose sand you can either blow it off, transport, and then add loose sand at the location; or write it in the sand at setup.