This kit contains 4 corner brackets, each comprise 2 side, inner and outer, along with 8, 1/4" stainless steel toggle bolts, washers and nyloc nuts.   Additionally, a #G (0.261") drill bit.   

The first step is to locate the tab of the inner bracket on each corner slot.   You will push the bracket all the way to the extreme, toward the top on one end and toward the bottom on the other end.    You will drill through the bracket into the box side.   Once this is done, feed one of the toggled bots into the first hole, then drill the second hole. 

 

It works best to remove the door fully from the box.  (Note, you can keep three hinges latched on either long edge, which allows the door to hinge on either side.  Have someone help, then add clamps to one side (left or right first, add the inner bracket first, feed the outer bracket with the toggle bolts from the outside, push them to the square hole to keep them from spinning.  Add a washer to each of the exposed threaded portion on the inside, add the nut, then tighten them all lightly.  Continue with the 3 other locations, then do a final snug tightening.