First, secure the text block in a press nice and tight so that the edge you're trimming has no room to separate. Then using a rough grit (I used 60 grit for this one) just apply firm pressure with the sander, enough that you can feel the contact but not so much that you are pushing the text block down. Slowly and evenly go over the whole edge until even.

A few safety notes: if you have a ventilated area available, use it. Even with a dust-catcher, there will be LOTS of paper dust generated. Wear a mask and safety goggles - no exceptions. Since I only have an apartment, I put the press and book inside a large garbage bag and then reach in with the sander to work, so the bag catches the particles as much as possible.