When I approach a security check in an airport, I often see some kinds of instructions in the front side to teach people what items I need to put into the trays.

It’s one of the critical tasks that require our full collaborations to ensure the safety for all of us. So from the perspective of airport staffs, they need to teach first-time travelers and also remind experienced ones.
Hence the simplest idea is to show some flyers or guidances in the front side of the security checkpoint, and hoping people would read them.