Early diagnosis and appropriate management of Cesarean scar pregnancy (CSP) are crucial to prevent severe complications, such as uterine rupture, severe hemorrhage and placenta accreta spectrum disorders. In this article, we provide a step-by-step tutorial for the standardized sonographic evaluation of CSP in the first trimester. Practical steps for performing a standardized transvaginal ultrasound examination to diagnose CSP are outlined, focusing on criteria and techniques essential for accurate identification and classification. Key sonographic markers, including gestational sac location, cardiac activity, placental implantation and myometrial thickness, are detailed. The evaluation process is presented according to assessment of the uterine scar, differential diagnosis, detailed CSP evaluation and CSP classification. This step-by-step tutorial emphasizes the importance of scanning in two planes (sagittal and transverse), utilizing color or power Doppler and differentiating CSP from other low-lying pregnancies. The CSP classification is described in detail and is based on the location of the largest part of the gestational sac relative to the uterine cavity and serosal lines. This descriptive classification is recommended for clinical use to stimulate uniform description and evaluation. Such a standardized sonographic evaluation of CSP in the first trimester is essential for early diagnosis and management, helping to prevent life-threatening complications and to preserve fertility. Training sonographers in detailed evaluation techniques and promoting awareness of CSP are critical. The structured approach to CSP diagnosis presented herein is supported by a free e-learning course available online. © 2024 The Authors. Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of International Society of Ultrasound in Obstetrics and Gynecology.