It really comes down to your comfort level, her personality, how long you've been together, and if you live together/have things at each other's place.
Essentially there are four steps.
Determine if you genuinely are ready to call it quits. You don’t want to create a yo-yo relationship where you breakup one day and call to makeup the next day. Take a sheet of paper and draw a line down the center. On the right side of the paper use the heading (pros) and on the left side of the paper use the heading (cons). List the pros and cons for staying in the relationship.
There are countless opinions about what is the right or wrong method to use. However breakups are done at the comfort level of the person ending the relationship. You must also take into account the individual’s temperament. Some breakup conversations have been known to go sideways ending in violence.
Always breakup at her place. This allows you the power to leave when you want to and it helps prevent her from having to drive away after getting the shocking news. Make sure to bring any of her belongings with you. Ideally if have items at her place you have gradually removed them prior to your talk.
Keep the conversation short and to the point. “I’ve given this a lot of thought and I have reached the conclusion that I need to end our relationship.”
Expect to be asked (Why?) however keep in mind there is no answer you can give that will put a smile on her face or cause her to feel you are making the “right” decision.
The only answer to the (why) question is, “I’m not happy being in this relationship and I understand you are not responsible for my happiness. That is completely up to me.”
Don't get sucked into a "blame game" conversation where you list all of bad traits. This prolongs the conversation and causes the other person to beg/plead and promise to change...etc It's not necessary to have a long drama filled nightmare in order to “justify” a breakup.
4. Move On and Allow her to Get Over You
Do not offer “instant friendship” as a consolation prize! This tactic simply raises false hope for your ex that if they remain in your life there is a “chance” they can win you back overtime. Unfriend her on Facebook and other social media. Refuse to respond to emails, texts, or calls. Don't acknowledge gifts/cards. It's a tactic to get you to contact your ex.
The best friendships between exes usually occurs after a large gap in time or when you both have found others.