I used to manage 54 employees for a branch that brought in $4 million in annual revenue. Unfortunately I had to fire people from time to time. I was once told that the way I did t it was "textbook". I actually gave classes to Managers covering this topic.
Here are some tips that might help:
-Do it first thing in the morning after everyone else has left. or if they are in the field call them back in.
-Don't let them know over the phone why they are coming back in when you call them- even if they ask, but don't lie and try to keep the conversation brief.
"Hey Jon I need you to come back to the office real quick"
"Am I going to get fired? Are you going to fire me??"
"Jon, stop that, I just need you to come in buddy"
-Whatever line you choose, the first sentence that comes out of your mouth should tell them directly that they are fired. Use the word "terminate", or "no longer employed" vs "fired". Be direct, say it in one breath. Say it without interruption. The idea is that you want to "shock" them (they probably already know they are going to get fired anyways), and you want to rip off the band-aid quickly. But above all, you want it to be clear. You don't want to have to explain that "yes" they are being fired. (the reason for "shock" is for your safety and you don't want to give them time to be able to rebuttal -hence why you don't tell them over the phone either") Have a 3rd party that is neutral on standby that you can "hand-off" the termed employee to.
"Jon, thanks for coming back, unfortunately, effective today, I can no longer keep you employed here....(pause)...before they can speak say, "George here will give you a ride home/George here will collect your equipment etc"
"Ok, Jon, because of xyz reason, I have no choice but to terminate your employment effective immediately"
"Ok Jon, unfortunately this isn't working out, so I have no choice but to end your employment today.
Don't say, "....I have to let you go...." because they will ALWAYS respond with, "what? are you firing me?" to which now your engaged in a conversation of clarification instead of termination.
-Don't blame "corporate" or higher ups.
-Take ownership of the decision -don't use "we", instead use "I". Don't say "Well my boss said I had to fire you." or .. "HR said I have to let you go", or "corporate gave me no choice" ...those are horrible and shows your weak and suggests that it wasn't your decision (which could open up the possibility of negotiation). It places blame on someone other than you. Firing someone is a serious decision and you should own it no matter what.
Keep it short. Get them out of the building as fast as possible. If you think there will be trouble, have someone else sit in the room behind them. Be respectful. Be encouraging.
Try to avoid saying, "I'm sorry".