Andy, if I'm recalling correctly, I think the way I used it was this (sorry I don't have time to make any diagrams or pictures at the moment):
Let's say you're going to trim the top of the mount in order to reduce the height. And this assumes you've already trimmed the width to the size you need.
First off, I used 2 small post-it's, one on left-hand side and one on the right-, wrapping them around so that the front and back side of the mount are secured together. That way, when you trim the top to reduce the mount to the height you need, everything stays together and doesn't get out of position.
Second, I then used that metal tool. There's one side of it that has that little notch -- just a little inset -- on one side of it. Kind of like what is shown, I think, in step 2 here:

The only difference is that I always did this with the cut side towards me, rather than away from me, like they're showing in the illustration.
Using that tool and in that position, I found that I could get a very even bead of glue on that cut. After putting down that line, I would then carefully put the clear front side of the mount back in place.
Also, unlike those illustrations, I did not dare try this with the stamp or souvenir sheet in the mount.