Cranioplasty treats and repairs cranial defects with a custom craniofacial implant (CCI). Typically, surgeons know the defect size prior to surgery. Recent efforts consider single-stage cranioplasty-performing the bony resection and fixating the CCI in a single operation. This paper develops a computer-assisted technique to perform single-stage cranioplasty. Intraoperatively, the surgeon traces the bony resection. The outline of the bony cuts is projected on a preoperatively-designed CCI to guide the surgeon during the resizing. A cadaveric case study showed good fit with minimal gaps between the implant and remaining skull. Moreover, the procedure reduced the time to resize the implant by an order of magnitude compared to manual resizing without the use of the computer-assisted technique. This approach represents the next step in quickly, effectively, and robustly performing single-stage CCI to treat craniofacial defects.