Every alter has to be approved by the Head Judge of each event it is used in, even if previously approved. The card’s art has to be recognizable, cannot contain substantial strategic advice or offensive images. The card’s name and mana cost may not be obstructed. Technically the power/toughness and loyalty areas can be covered but it is better to keep them visible.
Hurley Burley Studio made a slide show with pictures for her judge seminar presentation on alter legality, which you can find here.
But in the end the sweet thing about Alter Sleeves is that the worse thing that could happen is that you have to …. change your sleeve.