In terms of incompetantcy: Anything a normal human could fail at, a character has to roll a six-sided die to determine the outcome.

If they are untrained they need a 6 to succeed, or if it relates to a skill they need a 3+.

You can add points to a dice (you must declare you're using it beforehand) by spending willpower. 1 willpower = +1 to the dice.

The outcome of his incompetancy depends on the situation at hand.

I will say that if Jorde succeeds at a spell with a character, he no longer needs to roll for that particular spell with that spirit. In terms of whether Jorde remembers how to do it with other characters: generally no, unless that player has specifically stated otherwise (e.g. if Jorde is dying, a spirit might be like: I'll let the person currently in control access my memories of healing spells)