How does motor neuron disease progress?

Depends. There are many different types of motor neuron diseases, each of which has its own unique progression (which is how we tell which one you might have oftentimes). You'll need to be more specific in order to get a good answer. If you think you have a motor neuron disease, you need to get to the doctor right away.
Progression. The disease progresses as more motor neurons die off ultimately leading to paralysis and or stopping breathing. Als is the most deadly motor neuron disease.