I am NOT a personnel professional (HR to you kids), so this is a non-professional opinion. And my company was too small for HR - I did it all myself.
First of all, the time to start documenting performance, poor or excellent, is before things get to extremes. If I had a sense that someone wan't working out, I stuck a dated informal note in his personnel file for each incident of any consequence. Annual reviews are wonderful, but actually I never had a leaker last long enough to get to a six month review!
When I concluded that an employee wan't worth what I was paying them, it was frankly by gut feel, not my notes. But I felt it was important to document the reasons.
On the day of separation I'd call them in, tell them that my company wasn't a good match for their talents, and that they were no longer my employee. I had the file with the notes at hand, but not one employee ever asked to see them, and only one even asked for a reason for dismissal. (She was psycho anyway - interviewed well but only lasted a week). An employee knows when they aren't doing a good job, really.
In my opinion, the keys to dismissing an employee are:
Be prepared
Be professional
Be FIRM
Be courteous
Be BRIEF