At this point it should be obvious where the mirror is going to go. Attach it using your adhesive of choice, and it's time to start shooting.
Slide it on to your hot shoe and check to make sure it's not drooping down. If it is, you should go back a step and see if you can fix it.
If your measurements were right and you're lucky, the flash will be able to pop up and down with the unit in place (my flash just barely clears the right-angle bracket).
on the Lightscoop site are helpful. In brief, they suggest setting your camera to ISO 800, 1/200 sec shutter speed, widest aperture, flash compensation up as far as it goes, and the metering to spot. In reality, I've found the only really important setting is having the metering set to spot; if it's on matrix, the pictures will not come out. I keep the ISO at 200 and it works fine. You may need to tweak the settings a little.
It may not be pretty, but it gets the job done, and for $2 of purchased materials you can make four and give them to your friends!