Sprue cutters are basically tweezers with 2 cutting blades on the end. If you work with styrene they will
become indispensable.

Close up of the business end of sprue cutters. The two sharp blades come together when pressure is applied to make a clean cut in styrene.

Here is the recommended method for applying corner trim on Modeler's Choice styrene kits. Using plastic cement adhere the styrene strip to the corner of the building letting it extend over the edge slightly.

With sprue cutters snip off the over hanging strip styrene. If you do not own sprue cutters the overhanging strip can be cut with a razor blade or a hobby knife.

Most of the time no further clean up is necessary on the part you just cut. If it does require clean up a
quick pass with a small file will clean the edge.