Use an XACTO Knife to cut on the line you traced.

It can be difficult to control the knife when applying enough force to cut through the plastic, so I recommend going lightly for the first couple of times to make a groove. Once you have a properly-shaped groove, go back and apply more force to cut through the plastic. You can also drill 5 pilot holes using a fine drill bit at the vertices. This also helps to guide the knife.

When you are nearly done, flip the SIM-Card over. You should be able to see the outline of the cutout you made on the top side. For a cleaner result, finish cutting the slot from the bottom side.

Remove the piece you cut out of the SIM.