How can I trim veneer edging?
You really need a router.  Either a laminate router (the are small and easy to handle) or any router really.  Next you need a trim bit.  They have MANY different types.  Flush cut, 45, 25.5, finish bit.  The finish bit will leave a 1/16 laminate left to sand/file, or you could set the 45 bit to get almost flush.

Another way is with a sharp blade in your knife and a piece of wood.  Take a scrap piece of ply 4" wide and how ever long you need.  Place flat on the "face" of the cabinet and let about an inch hang over the edge.  We are using the wood as "backer" when we score the laminate.  Now take the knife and score the underside of the laminate a few times pressing the knife to the laminate and the lam to the wood, using the edge of the cabinet as your guide.  Do this on a practice piece to get "a feel for it".  Either cut all the way through, or score enough and just snap the lam.

* Edit *
If you do use a router, apply 3/4 masking tape (or blue tape) to whatever surface the bearing will ride on.  This will prevent any marks the bearing will leave, mostly a "shiny" line.