A method of manufacturing a customized patient-specific orthopaedic surgical instrument, the method comprising: generating a three-dimensional model of a patient's bone based on patient-specific data, identifying a first region of the three-dimensional model of the patient's bone, defining an outer boundary of a customized patient-specific surgical instrument on the three-dimensional model of the patient's bone, generating a customized patient-specific surgical instrument model within the outer boundary, the customized patient-specific surgical instrument model comprising a bone-facing surface, and fabricating the customized patient-specific orthopaedic surgical instrument from metallic material based on the customized patient-specific surgical instrument model, wherein generating the customized patient-specific surgical instrument model comprises: (i) generating an outer surface of the customized patient-specific surgical instrument opposite the bone-facing surface, (ii) extending a guide body outward from the outer surface of the customized patient-specific surgical instrument model to a free end, and (iii) defining a guide slot in the guide body, the guide slot being sized and shaped to guide a surgical tool into engagement with the patient's bone; wherein the bone-facing surface includes a customized patient-specific negative contour [338, 354] that is configured to receive a corresponding positive contour of the patient's bone, characterised in that the generating the customized patient-specific surgical instrument model further comprises: (i) defining a cavity in the bone-facing surface over the first region of the three-dimensional model of the patient's bone, the cavity having an outer edge aligned with the outer edge of the first region. generating a three-dimensional model of a patient's bone based on patient-specific data, identifying a first region of the three-dimensional model of the patient's bone, defining an outer boundary of a customized patient-specific surgical instrument on the three-dimensional model of the patient's bone, generating a customized patient-specific surgical instrument model within the outer boundary, the customized patient-specific surgical instrument model comprising a bone-facing surface, and fabricating the customized patient-specific orthopaedic surgical instrument from metallic material based on the customized patient-specific surgical instrument model, wherein generating the customized patient-specific surgical instrument model comprises: (i) generating an outer surface of the customized patient-specific surgical instrument opposite the bone-facing surface, (ii) extending a guide body outward from the outer surface of the customized patient-specific surgical instrument model to a free end, and (iii) defining a guide slot in the guide body, the guide slot being sized and shaped to guide a surgical tool into engagement with the patient's bone; (i) generating an outer surface of the customized patient-specific surgical instrument opposite the bone-facing surface, (ii) extending a guide body outward from the outer surface of the customized patient-specific surgical instrument model to a free end, and (iii) defining a guide slot in the guide body, the guide slot being sized and shaped to guide a surgical tool into engagement with the patient's bone; wherein the bone-facing surface includes a customized patient-specific negative contour [338, 354] that is configured to receive a corresponding positive contour of the patient's bone, characterised in that the generating the customized patient-specific surgical instrument model further comprises: (i) defining a cavity in the bone-facing surface over the first region of the three-dimensional model of the patient's bone, the cavity having an outer edge aligned with the outer edge of the first region. (i) defining a cavity in the bone-facing surface over the first region of the three-dimensional model of the patient's bone, the cavity having an outer edge aligned with the outer edge of the first region.