Spread the jaws on your vise and lay the lockplate on the surface, with the tumbler projecting into the space between the jaws, then use the punch to drive the tumbler out from the cock. Use a good strike and it'll come right out. Be ready to catch it with something under the vise, and be careful not to lose the fly, or it will get lost under the work bench (I think that's a rite of passage). It's good for this to be tight - don't file this any looser.
-Eric