I have always found making godets to be a real headache. These two videos give a good explanation of how it's done, though I'm still not totally clear on how to get the sides of the godet the right size!
The important thing to note is that a godet is never a triangle: the hem is always slightly rounded. Some people start with a quarter-circle, which means the hem needs a lot of adjustment when you're finished, because it hangs down a long way in the center. The dressmaker in these videos makes a much shallower curve, but doesn't explain how to make sure the sides are going to fit the slit.