Pinning

Always use the radiolucent Mayfield. Single pin on the ipsilateral forehead. Double pin in the axial plane along the dependent part of the occipital bone. 

Positioning

Lateral position with the dependent arm wrapped in purple foam and allowed to rest in the angle of the Mayfield. 

Tape the shoulder back to the edge of the bed before wrapping that arm in purple foam and wrapping around the bed a couple times. 

Flex the neck slightly then rotate so that the patient's gaze is towards the floor. 

Marking

Start at the mastoid tip and draw a line along the expected location of the sigmoid sinus. Begin curving medially (i.e. towards the occiput) above the level of the transverse sinus. Take this line medially until you reach the external occipital protuberance, then curve down in a straight line along the spinous processes to the spinous process of C2.