First, go in a wood near you when sunset is near only with a fountain pen, a piece of paper, a needle, a strand of hair of the girl, a red candle and a mirror.
Now you need to find a stump of a tree, don't worry: there will be one in every wood you go. To do this light the candle and hold it in one hand, with the other hand close your eyelids and then put the palm of the hand where your heart is. When the candle stops burning you will feel an electric shock and hear the candle burn out and that means it is sunset, so then open your eyes and you will see a tree stump on your right. If it is on your left it means you have failed and you will be already dead when you notice. If it is on your right take the mirror and you will see a demon in it, the one you would seal a deal with in the ritual. If you don't see him then close your eyes again and you will have the tree stump on your right. If you see him then smash the mirror on the tree stump.
Then put the things you have brought on the tree stump and dip the strand of hair of the girl in the candle wax.
After take the pen and the paper and write on the paper the name of the girl, after you did that you have to fold the paper in half and open it again, then take the needle and put a drop of blood from your right index finger on the paper, on the other side, then put the strand of hair on the blood drop and close the paper.
After that you have to say "O, amor qui nobis signa haec multum placet 'amor aeternus." and light the candle again, then burn the paper, tricking the spirit that sealed the deal and turning him to ashes, then blow the ashes away and leave the candle to burn out.
The next day the girl you love will try to kiss you. If you don't kiss her, the spirit, or I should say demon will appear and send you to the underworld. If you accept, the deal is sealed, and you will be in love Forever... Because you wouldn't want to Dump a Demon right?