It's a little nerve wracking to be sure - but I found a You Tube video that worked for me... I just used heavy duty clippers (sheet metal, I believe) and clipped the upper 2 corners far away from the card itself.
Once that was done, I pry the slab open using a flathead screwdriver, again, far away from the card.
I think I've only opened 2-3 slabs in total so I'm hardly an expert - but this worked well for me. Good luck!
(btw, the outcome for me was a bummer - I was SURE that my 56 Hank Aaron was incorrectly graded! And I was RIGHT! It went from a grade 4 slab into a nice new 3.5 slab
)