When you learn a new spell and have time to sit and meditate for a few hours per level on its arcane mysteries roll + INT:
On a 12+ you learn the spell and get to add a detail that makes the spell uniquely yours (to be decided by the player).
On a 10+, you learn the spell without a hitch.
On a 7-9 you learn the spell but there is a new cost or problem that now comes from casting the spell (to be decided by the GM).
On a miss, you learn the spell but it has a disastrous side-effect that now goes along with it (to be decided by the GM).