How do you secure a cargo in a shipping container?

It depends on what cargo you are shipping which determines the container type in which you are shipping it.

Let's look at the containers here:

A flat rack conveys heavy duty equipment that is over height and over dimensional. The sheer weight of such cargo alone keeps them secure from tipping, and the foreknowledge that they will be shipped “open” ensures they're well coated to shield them from the elements during sea passage. To secure some component parts they can be fastened to the main plant.

A reefer container conveys refrigerated cargo that has to be shipped under regulated temperature. Poultry and diary products and fruits are usually packed in cartons which are neatly stuffed inside the container from floor to roof and side to side. The even dimensions of the packaging guarantees their stability with no extra fastening. Cartoned reefer cargoes are a surveyor's delight due to their ease of handling.

Bulk cargoes like grains are a bit more treacherous to secure than the two classes earlier mentioned. Grains shift of their own accord. They can combust after surpassing certain heat threshold. To secure them you make use of the dunnaging agreed in the sales/purchase agreement. For instance you could dress the floor and walls of the container with brown papers, cardboards, wood or bamboo plies. You add “dry bags” after stuffing to absorb moisture and reduce accident of inherent vice. You consider the VGM (verified gross mass) so you don't “overload” .

General cargoes like spares and similar portable items are also packed in cartons but stacked on pallets inside the general cargo container (20 ft, 40 ft regular or 40ft high cube, HC). Again the overall gross weight is considered to avoid accident at sea and during intermodal transportation.