Switch the positions of the outside and inside hands on the oar handle, so that the arms are crossed. The outside arm should cross above the inside arm. Take straight-arm strokes only, to avoid needing to deal with bending the arms at the finish.
Note
The aim of this drill is to emphasize the different in outside and inside shoulder heights at the catch. By putting the outside arm over the inside arm, the outside shoulder should be forced to be slightly higher, and cannot dip at the catch.