Documentation is part of supervision. You start by laying out what kind of behavior and performance you expect.
Check for understanding. Don't trust communication to chance. Make sure that people understand what you tell them in the way that you mean it.
Make small corrections along the way. Most good supervision happens in the cracks in the system. Most people who work for you will change their behavior if you suggest they do so.
If they don't change their behavior, make sure you understand the problem. What looks like a behavior problem might be a resource problem or a training problem. Make sure your people can do what you want before you hold them accountable for performance.
If your subordinate can do the job, but isn't, let him or her know that you're going to start documenting their behavior. That's not particularly difficult, but it takes time and diligence.
Make sure you document the behavior of anyone you might have to fire. Start by understanding the documentation basics.
You document so that you can explain your decisions to other people at some time in the future. If your subordinate challenges your actions, that questioning can be aggressive and adversarial.
Document behavior. Behavior is what people say and what people do. Nothing else.
Describe the behavior using objective language. I call this the "Joe Friday Rule," just the facts. Leave out the adjectives.
Write up your documentation as soon after the behavior or counseling session as you can. Within 24 hours is good. Before you go home is better. Right away is best.
The closer you do your documentation to the behavior or incident you're describing, the more likely you are to remember details and get things right. And, the more likely you are to be able to defend your actions and descriptions later.
Include the important information. Who was involved? What happened? Be specific about dates and times.
It's easier to do good documentation if you do it the same way every time. I recommend that you use a simple form to help you do that.