In recent years, patient specific instrumentation (PSI) has shown to be a promising application of 3D printing in orthopedic surgery. By aiding the surgeon with accurate placement of instruments such as saws or drills, surgeries can be performed faster, and require less surgeon experience. While most of these devices interface directly with the bone, invasiveness can be greatly decreased by applying the device to the outside of the body. Such an external device also allows for guidance with respect to bones that are too small to place a device on directly, for example the carpal bones in the wrist. In this thesis, an external PSI is designed to aid surgeons in accurately drilling a tunnel through the scaphoid; one of the carpal bones. The main challenge addressed in this thesis is how to rigidly attach a guide to the largely soft and elastic exterior of the hand. This was overcome by ensuring that the guide only contacts the hand at locations where the bone lies directly under the skin. Exact constraint design was applied to determine a minimal set of such contact locations that fully constrain the relevant anatomy. A 3D printed prototype was made for 6 participants and measurements were done to determine the positioning accuracy as well as the stiffness of the connection between the guide and the body. The results show that external PSI is a promising technology for application to wrist surgery, but may not be applicable to other parts of the body due to a limited number of suitable contact locations.