How does double-wrapping work?

EVE University has a tutorial here see the 'Advanced Hauling' section and this item specifically:

If you put your goods in a secure container, and then make a courier contract with that container, then the items in the container will be "double wrapped", and protected from cargo scans

Unless they've changed something recently you specifically can't put a container inside another container, if you try you'll get a message about Planck something or other.

You use double wrapping when you want your cargo to be unviewable from gankers cargo scanning the hauler ship.

To double-wrap something, you need to have an alt or another character who you trust that you can make out a contract to.

The base idea we're exploiting here is that you can't scan a container inside a container with cargo scanners. When you make a courier contract, it puts all items you've attached to the contract in a special container called "plastic wrap". All you need to do is make sure all the items you want transported are already inside a regular container (of a size that you can fit into whatever you're hauling with), and then you make the courier contract and attach that regular container to the contract. The regular container should now be inside the plastic wrap container. Make the courier contract a private one to your alt or trusted friend, they can then accept the contract, receive the plastic wrapped container, and either haul it for you or just regular station trade it back to you if the character isn't on the same account.