1. Write out everything you want to say.
Grab a pen and paper and write out everything you want to say to the person in question. Write as though you are addressing the person directly, and as though you're having the conversation right then and there. And when I say write out everything you want to say, really write out everything, in detail. Express all your thoughts, feelings, decisions, etc. in as honest and raw a way as possible. You may not actually say everything you write down when you have the conversation, but doing this will help you sort through all your thoughts.
2. Talk it out with someone else.
Find a close friend or family member to talk things out with. If possible, someone who has firsthand knowledge of the situation or the person involved would be ideal. Talking to someone about difficult situations is always helpful because it allows you to get an outsider's perspective and insight. So I recommend talking to a trusted friend about the issue you're having, and, if you want, you can even use that friend to practice what you're going to say to the person you need to have a conversation with.
3. Do some prayerful preparation.
Prepare to have your tough conversation by taking it to prayer. Ask the Holy Spirit to provide a good opportunity for the conversation to happen, and to inspire you with the right words to say. Pray for strength and courage to have the conversation, and for a good outcome. If you entrust the situation to the Lord, He will guide you through it, bring peace to your heart, and resolve the situation according to His will (which will mean the best possible outcome).
4. Find a good time to have the conversation.
Find a time when you and the person can be alone and have some privacy. If possible, make it a time when you and/or the person can or has to leave shortly after the conversation. For instance, pick a time when either you or the person has to be somewhere relatively soon. This way you won't be stuck awkwardly hanging around after having an uncomfortable, brutally honest conversation.
5. Take a deep breath and just say it.
As hard as it may be, just spit it out, whatever it is you have to say. It might take a few deep breaths first and maybe even some tears (in my case... I'm a crier), but when it comes down to it, you just gotta say it. Once you do, you'll feel a million times better and as though a weight's been lifted off your shoulders.