Stand with a tissue on your head, turn around 6 times quickly, and while you are still dizzy intone, "Goodbye. Goodbye. I am breaking up with you now" 6 times. Then, when you have ceased to be dizzy, go over to the house of the person with whom you want to break up with and tell them straight out. As you walk away from them, clap your hands 6 times, and the spell is complete.
Generally if you don't know how to put together your own spell, then you probably aren't ready to be casting them. Having said that, the following might be useful. Keep in mind, that "break up" spells are the same as "love" spells (just reversed). You're still messing with someone's free will, especially if they aren't the one that wants to break up. If you still want to use it I would add a note (*) allowing for "if it is meant to be, then all will work out in the end". Personally though, I would just break up with someone the regular way. Sure...it can be a hard thing to do, but sometimes you just have to do it.
our path's diverged new lives to start what once was merged here now must part with blessings bright we'll start anew the time is right we know it's true *(see note) As I will, so mote it be.
Normally "ending" spells are done with the waning moon, but since this one is focused on starting fresh, you could do it with either. It's fairly simple - say it like you mean it. Also, either have a picture of the person (or a clear image of them in your mind) while you say the words, and visualize the two of you going your separate ways.