It sounds simple; dump a bag of crushed ice into the ice chest, put in the food and drinks and then if you are really serious, layer another bag of crushed ice on top. This works fine if you are going on a picnic, but simply tossing a couple bags of crushed ice into the ice chest is completely inadequate for even short weekend camping trips.
The key to an ice chest is, of course, the ice. But the key to ice is mass, since an object with greater mass will hold its temperature longer than a similar object with less mass. For this reason a big block of solid ice, instead of hundreds of small cubes of ice, will last much longer, even if the block of ice and the bag of crushed ice are the same weight.