Assembly is pretty basic if you refer to the schematic drawing. If I had paid closer attention to the size of the battery holder vs the inside of my box, I would not have had to do some work with my dremel tool to grind down some inside corners of my box to have things fit properly.
I used a piece of double sided tape to attach the small relay to the side of the battery holder. These two parts then fit snugly into the case. I then soldered the rest of the wires to the exposed connections on the relay. Your relay should come with a diagram showing which pins are for which purpose.
I then made a small notch in the side of the case for the input / output power cables to exit. A small cable tie on the inside of the box provides strain relief for the cables.