In marichyasana C, the non-marichyasana leg is straight. You twist and bind the marichyasana leg from the outside.
This pose can be challenging if your thighs are short compared to your body. One way around this may be to work at pressing your shoulder down. If twisting to the right (with the left leg in marichyasana) the work at pressing your left shoulder down to making binding easier.
Assuming your are twisting to the right, (and assuming you haven't got the above mentioned issue) the first step is to get your arm to the outside of your thigh. This by itself won't be enough to get you to bind but it is a first step.
In this position work at getting your shoulder down so that your elbow is at or past the front of your shin. In this regard this pose is similiar to grabbing a wrist and binding in Ardha Matsyendrasana. However, instead of passing the forearm between the shin and thigh, you are trying to wrap it form around the front of the shin.
Once your arm is reaching far enough forwards you can use your other hand to try to push your forearm to the outside of the shin and thigh. Or work at using your arm muscles to internally rotate the arm so that this happen.
Note here that the arm action you are trying to do in order to bind (internal rotation with the elbow to the side or even forwards of the ribcage) is very similiar to that used in the penguin shoulder stretch.
Once you get the forearm under your thigh, try to hook it against the thigh so that you have some stability. You can then reach your other hand behind your back to try to grab it.