Make sure you are not being disrespectful and playing the blame game. You do not have to bring up the mistakes he or she made. Instead, you have to tactfully present your opinions and why you think it is not working out. You have to be honest and not disrespectful.
Say positive things about him or her to make sure they are not hurt. This way, you will make them realise that even though they did their best and you appreciate it, still things are not the way you would like them to be. For most partners, appreciation matters a lot. You do not have to show them that you never appreciated them and that it is entirely their fault.
Be calm and patient. Do not scream or yell. You do not want to lose the respect your partner has for you. Make sure you explain things to them in a very peaceful manner so that they understand it easily. Tell them how much you respect them and like them but being together will only hurt your friendship.
Instead of asking for a breakup, ask for a short break. This works best. Ask your partner that it is about time you both figure out if you want to take the relationship any further. Ask for a short break for a week or two. Ask them for time. Later on, if you feel you are not up for it, you can tell them that it is not working out.
Make sure you give them enough time to recover. Do not impose your decision on them and walk away. It is a mutual decision and it should be agreeable by both parties.