First time firing an employee, any tips?

Y'all I've been a manager at this smoke shop for maybe two months and Ive been task with letting an employee go on Tuesday and I have NO idea what Im gonna say.

They're getting let go because they all of the sudden started being rude and uncaring about the customers and we've had complaints. It was also told to me by my boss who was there at the time that she yelled at her dad in front of the store loud enough to hear through the glass. All of this was done directly in front of the owner who confronted me with this task.

Anything helps!! I have no clue what im doing here this is new territory