Importance: Three-dimensional planning software is not standardized in facial gender-affirming surgery. Objective: To develop and validate surgical planning software to create cutting guides to contour the lower jaw border. Design, Setting, and Participants: A 3-year prospective case series study done in three phases: software development, validation, and surgical guide application. Ethics committee approval was obtained to enroll the patients (Clinical Research Ethics Committee, Hospital Costa del Sol, Marbella, Spain). Main Outcomes and Measures: Validation phase: degree of agreement between the planned and obtained results, modification of cephalometric parameters, and surgical times. Application phase: surgical technique description, complications, and patient-reported outcome measures. Results: The degree of agreement between the planned and obtained results was inframillimetric (0.31 ± 0.70 mm). The guides reduced the mandible to within feminine parameters (p < 0.05). Surgical times decreased by 10.96% with chin ostectomies (p < 0.05) and 23.06% with lower jaw border (angle-to-angle) surgeries (p < 0.001). In the application phase, revision surgery was required for 11 patients out of 260 (4.23%). Conclusions and Relevance: The use of cutting guides on the lower jaw border is effective, helps reach standard feminine parameters, and decreases surgical times.