Proving mutual exclusion

What are the steps we need to show to prove mutual exclusion condition is satisfied?

1. Bring Process Pi to the critical section. 
2. Try to bring in process Pj to the critical section. 
3. Show that Pj cannot enter the critical section so long as Pi is still in the critical section. In other words, show that Pi exiting the critical section and reaching its exit section is a necessary condition for Pj to enter the critical section. 

This shows that two processes cannot be in the critical section at the same time which proves mutual exclusion.

Thanks to http://proto.cs.qc.cuny.edu/cs715/ConditionsCriticalSection.pdf