Here's a bit of advice that I give to clients who have a problem employee:
Boil your description of the problem down to one sentence. If possible, reduce it to one word.
The reason for this exercise is that it will force you to clarify the situation in your own mind before you talk to the employee about it. Far too frequently, managers and supervisors call in the employee and then either recite chapter and verse of everything that has gone wrong in the past six months or grumble something about "unsatisfactory performance."
Contrast that with: "Your job performance is unreliable. On occasion, it is very good, but other times it is far from that and I need you to perform in a reliable manner. Let me give you some specific examples."
The session proceeds from there. Everything that is cited pertains to the reliability problem and the ensuing action plan is reasonably designed to correct that problem. By the time the session is completed, the employee knows exactly what needs to be addressed.
Resist the temptation to throw in a hodgepodge of items. Look for a central theme that will accurately describe the conduct that needs to be corrected. Don't let personality slip into the discussion. Stick to performance.
By first being clear to yourself, you will enhance your ability to be clear to others.