Hold the oar handle only with the inside hand. The outside hand can be placed on the outside thigh or hip. Be careful that placing it behind the back can lead to incorrect rotation of the upper body. Now take regular strokes. You can start on the square, and then transition to the feather.
Note
The oar loading will feel heavier with just one hand on the handle. Focus on feeling the hang through the lats on that side of the body, and holding good posture from the catch through the drive.