The rules state that you also need some gestures and ingredient for the successfull completion of the spell, but I usually don't bother with the ingredients part. There is also a rule about if you have less that 12 magic points left meaning that spell casting is made much harder.
After expending the magic points I think a roll should be made, depending on your Int/WP and the level of the spell in relation to your Magic Level. I think that the rules state automatic success (as long as you known the spell in the first place), but I'm not sure what I should really do here. I think it depends on the players to some extent.
There are also other rules obout know spells, etc, but they're pretty boring. So that's all I'm going to write here.