To make a new spell you must make a knowledge check of the appropriate skill equal to the DC of the spell you wish to create The DM an the player must agree on a reasonable DC. Making a new spell from scratch however is difficult and takes 3 hours for every point of the DC, due to having to understand the theory behind it, and make sure no ill effects upon casting. Exceeding the check by 10 or more represents a breakthrough in your research counting the time that you have researched for that day as twice as much. If you make the check you progress normally, for the rest of the day you decided to work on it.
Let's say you want to make a new spell in Curses and Counter-Curses. You want the spell cause them to suffer a wave of nausea making them suffer from the sickened effect. You decide to research it by making a Curse and Counter-Curses check. Making a new spell from scratch however is difficult and takes 3 hours for every point of the DC, due to having to understand the theory behind it, and make sure no ill effects upon casting. Make a knowledge dark arts check equal to the DC of the spell that you wish to create. The DM and the player must agree on a reasonable DC. If you make the check you progress normally, for the rest of the day you decided to work on it. Exceeding the check by 10 or more represents a breakthrough in your research counting the time that you have researched for that day as twice as much. The DC for this spell would most likely be a 20 and would require a ranged attack roll, and a Fortitude save on the person who is being targeted.