This step can be a little difficult, but there is only one real word of caution that you need heed in order to get it right: TAKE YOUR TIME. Do this step slowly. Sand and file a little tiny bit at a time. If you sand off too much, you cannot go back. Be patient. Take it slow.
Get the message??
For this step, we're going to be using the body assembly, and the little acrylic rod that came with the kit. Your first step is to test fit the rod. Eyeball it against one of the reference photos in this tutorial to figure out where it should fit. This will give you an idea of how much you need to file off.
All you really need now is a file and some sand paper. Heck, you can accomplish this with only one of these.
After a little work and some patience, your rod will be ready to glue. The top of the rod should be flat, and the bottom should be angled to match the angle on the button holder of the pass. It doesn't need to match the curvature, but hey, if you want to get that crazy about it, go right ahead.
For putting the rod in place, I recommend some epoxy cement. Get the quick set stuff. I like to use a pair of tweezers to set the piece in place. I mix my epoxy using popsicle sticks. I do the actual mixing on a stack of post it notes. They don't take up much space on the work bench, and when the glue cures, you can just peel off the top piece and throw it away. Very efficient!
Another pro-tip here is to use a twisted/broken popsicle stick to actualy apply the glue. Toothpicks work just as good, though I did not have any on hand. The great thing about a snapped stick is that you can also use it to scoop up excess glue that you may drop on the body.
Once you have some glue mixed up, apply it to the body. The kit has two little pockets where you can shove some glue. These pockets also indicate the placement of the rod. The idea is that if you get some glue in the pocket, it will hold onto the body a little better.
Using the tweezers, put the rod into place. You will need to eyeball the placement to make sure it is sitting not only horizontal to the size, but is also seated properly along the thickness of the body. If you screw this part up, you can always pull it out and do it again. This is one of the main reasons I prefer epoxy over super glue.
Once the glue has dried, you are all set!