Option 1 - This process that has worked for me;
-Mark the top surface of each skin "NOT THIS SIDE!". You want to sand the under side of the skins.
-Glue sand paper to a flat piece of wood (preferably 3/4" or thicker). Make a couple sanding blocks.
-Secure skin to workbench so the area you plan to sand down is on a corner of the bench.
-Protect the area you will not be sanding with a diagonal strip of tape.
-Use some pieces of aluminum the same thickness as the skin for spacers (.032 for the inboard skin, .025 for outboard skins most RV models), if for example the taper area is 1" x 1", clamp the spacer sheet 1.5" x 1.5" diagonally from the corner.
-The idea is to sand both the spacer piece and skin in the same motion. This establishes the taper angle. Light even pressure.
-Don't take too much material out, ideally about 1/2 normal thickness along the edges, avoid sanding down to a knife edge.
Of course all this happens before you dimple these skins & should be noted that care most be applied when finally dimpling these thin portions.
Option 2 -
-mark & protect skin as above
-use a big flat file & SLOWLY work towards desired taper
-finish off with fine grit sand paper block
Not as much control with this process but what ever works for you.