There is a time when you are managing people that you have to sit down, look them in the eye and say something they don't want to hear. It is the hardest part of management, knowing that what you are about to say may not be received well. Usually this is around a topic or situation that needs to be addressed and its not comfortable to address it.
So how do you overcome this and give both of you the best chance of success with whatever the issue is?
First is to make sure you have clearly defined the issue at hand. Spend time defining the problem with as much factual data as possible. If its all anecdote then write it down and work out the details. When you present an issue to a person they will most likely challenge you on it in some way. Having the data to back you up is critical to the conversation. If statements are vague then the problem feels vague and the person will just put up walls of defense and stop listening.
Second is to understand what you want the outcome of the conversation needs to be. Are you looking to change a specific behavior? Is the goal bigger than that? Always tie some element of data to the ask. That way if the person isn't clear on the "why" at least they can understand the "how." Having a data point that both of you can reference also takes out some of the emotion of the conversation. It's impossible to manage people without emotions, especially when there is a problem that needs to be addressed, but if there are mutually agreed upon points that both of you can reference it makes progress more likely.
Finally when reviewing the results with a person make sure to highlight the positives. People want to be acknowledged for progress. When talking about something that is continuing to be an issue even after having that difficult conversation you need to figure out what the real root cause of the problem is at that point. Assuming they understood the measure that they would be help up to why didn't they achieve it?
Take all factors into consideration when trying to change a behavior of someone you manage. Be open and watch their body language. Go off your script if you need to, your employee will appreciate it.