Roll d6 to see what fumble does:
1-2 - attack is missed and balance is lost. Loose next turn.
3 - attack is missed and character staggered. Loose next turn and AC like hit from behind (or similar).
4 - attacker looses grip and drops his weapon.
5 - attacker manages to wound himself. Roll damage and divide it by two. Round up.
6 - attacker manages to wound himself as above and looses balance like in 1-2 roll.
I will use this in my LotFP game. (natural 1 is fumble and natural 20 is roll damage dice twice).