Fair and final firing starts with good supervision. Good supervision creates, shares and checks clear and reasonable expectations. Good supervision checks in a lot to see if performance and behavior match up to expectations and coach, counsel, and correct. Good supervision warns an employee who is not meeting expectations that his or her performance will be documented and that if sub-standard performance or unacceptable behavior continue, it could result in firing.
You may not think the above is about firing, but it is. If you don't set expectations and monitor behavior and performance you can't make a serious case for dismissal, even if you have good reasons.
Your documentation should be about specific incidents. You should describe the behavior or performance in straightforward language. I call this the Joe Friday Rule: just the facts. Leave out the adjectives. Leave out references to attitude or motivation.
You do good documentation so that you can justify your decision to someone else at a later time if that becomes necessary. But there are other benefits, too.
The frequent contact makes it clear to you and your employee what the situation is. That often leads the employee to seek other employment without the need to fire.
Let's also be clear about something else. If you've done a good job of hiring, most employees will do what they're supposed to do and work to achieve the levels of performance you require. For the small number that don't, notice that you're going to document will usually do the trick. In other words, if you do hiring and supervision right, you should have to fire someone in only the rarest cases.
But what if this is one of the cases? Then here are a couple of tips.
Fire on Monday, so your employee can get right to the task of job hunting. Don't give him or her a weekend to stew about things.
Fire in private. Begin by, slowly, reviewing the incidents of poor performance/bad behavior and all the sessions you had to discuss them.