One of the most difficult conversations to have with someone is telling them that they are not doing well in their role. If they are not very self-aware and have been given different feedback prior to that it is even more challenging. It is going to be a bit of a shock to them and they are going to be feeling very defensive. The best thing that you can do is to be honest with them and discuss how they can address the situation.
Although it is difficult, it is very important to share examples, feedback from other sources and how you will partner together on actions to change the behavior. These are concrete things that can reinforce the feedback that you are giving to them. Being able to share the expectations that you have of them and of the role is necessary. You can use competencies and a job description to show where they are doing well and where the opportunities exist. It will also be helpful to them as they are designing a plan for improvement that includes actions to demonstrate improvement.
This is a change that they need to make, but they will need your support. Offer consistent communication to reinforce positive progress and redirect if necessary. Just make sure that they are owning the work that they have to do and you are there to provide feedback. It is a challenge to go through this situation and it will test your leadership. Trust yourself and you will be able to look in the mirror each day knowing that you are doing the right thing for the best interest of the company and the employee.