The Marichyasanas are powerful, important postures because the rotation of the spine causes the muscles of the torso to stretch on one side as they contract on the other, ultimately improving symmetry between the right and left sides of the body. Regular practice of these postures develops a toned waist and a straight, strong spine.
Marichyasana C is a favorite of mine in personal practice, particularly for addressing discomfort in the back and/or tension in the external rotators of the hip.
Begin in Dandasana (Staff Pose). Bend the right knee so that the shin bone is perpendicular to the mat with a 4-6 inch space between the right foot and left thigh. While not part of the "official" instruction for this posture, it may be useful to turn the right heel out slightly, encouraging internal rotation of the leg, and slide the left hip forward to "unsquare" the hips in preparation for the twist. This ensures that the greatest degree of spinal rotation will be directed toward the thoracic vertebrae and kept out of the lumbar, which is not well suited to twisting.
Once the placement of the legs has been achieved, wrap the left arm around the right knee with an internal rotation of the shoulder. Take the right arm behind the back and clasp the hands together or grasp the right wrist with the left hand to bind the arms, as shown below (opposite side pictured).
Keep your weight forward and aim to close the gap between the torso and your right thigh. Ground through the inner edge of the right foot and use the leverage of the bind to twist more deeply. The straight leg remains active with enough internal rotation to hold the foot perpendicular to the floor. Both sitting bones stay in contact with the mat.
If it is not possible to bind the arms, you may modify the pose by placing the right hand on the mat behind you while hugging the right knee with the left arm. Focus on grounding both sitting bones and sitting as upright as possible.
Stay for 5-10 breaths and repeat on the opposite side.