I use a new single edge razor blade or a utility knife with a new blade to trim the veneer to between 1/16 - 1/8 inch from the edge. When trimming, cut "uphill" in relation to the direction of the grain, so if the veneer splits, it will split away from the edge. Don't slice down through the veneer--this tends to make the veneer fold down--but keep the blade perpendicular to the veneer you are cutting and pare from one end of the veneer to the other. Don't try to take all of the excess off with one stroke, but take it off in slender slivers, letting the cut follow the grain of the veneer away from the edge.
I then use 220 or 150 grit sandpaper wrapped tightly on a sanding block and carefully sand away the remaining excess veneer. When sanding, orient the sanding block the same way as the razor, perpendicular to the veneer edge, and sand in one direction only, again uphill in relation to the grain direction. Use the surface you are sanding down to as a guide and use long, slow strokes from end to end.
I hope this helps.
--------------Mark