You can usually do it by hand (or use a small screwdriver).
GENTLY bend the outer case near a corner...this should allow the inner piece to "pop" out at that corner, then do the next corner. A fingernail under the inner corner and pull the outer case carefully away at that corner is usually the best way.
Putting a case back together is similar...the new piece should "pop" back in. Just be careful that you have the paper sections correctly lined up first.
Check the picture in MciahelÂ´s link to see what the pieces should look like.
DonÂ´t worry too much if you break one...theyÂ´re cheap. Just try again.